Our weekly masturbation sessions had a really nice vibe to them. They always started with solo pleasure while watching each other, then we would naturally pair off and have sex.

Samantha and my best friend Tiff were now girlfriends, while Jack and Steven were boyfriends. I'd recently brought in Thomas to make up an even number of six. We weren't a couple, but we'd had sex a few times. Occasionally, some of us would play with the others: Tiff might have sex with Thomas, and I loved to swallow the boys' semen.

The only time I missed a session was when my parents took my brother and me on holiday. I discovered that Thomas hadn't shown up in my absence, and the other group members had assumed it was to leave an even number again. By all accounts, the other four still had a lot of fun together.

But when he still didn't turn up the week I returned, we wondered what was wrong. When he eventually answered our phone calls, he finally told us.

Ever since our masturbation sessions in the school changing rooms, he'd had difficulty orgasming. While the other boys – and most of the girls – only took a couple of minutes, he often didn't finish at all, and he was becoming embarrassed about it.

I made it clear to him I actually liked how he took longer. He could stay hard for what felt like hours, thrusting in and out and giving me a chance to have orgasm after orgasm. It was even better for Tiff, who was able to orgasm by penetration alone, whereas I needed my clit touched. Steven, on the other hand, would last about five minutes maximum, with barely an opportunity to get off even once.

We started our session with five people anyway, believing we hadn't convinced him, but the doorbell rang an hour later and there stood Thomas. We offered him a chance to prove how much we enjoyed his long sessions. He would wear the same condom with me, with Tiff, and then with Steven.

I had two orgasms, with help from Samantha holding my vibrator against me, and Tiff also had two without the vibrator. Just before moving on to Steven, Tiff turned to Samantha.

'I know you're a lesbian, but do you not want to try it? Just once?'

From the expression on her face, it seemed for a moment she might agree, but she said, 'No. I'm sure it's great, but not for me. Just move on to Steven.'

Another great thing about Thomas is how he'd discovered his love of other boys in the group. Jack had eventually admitted he was gay, and Steven was already openly bi, and it was always a treat to watch male-on-male action. Jack didn't like to be penetrated, but he helped Thomas to enter Steven. It was fun to masturbate to the sight of them all kissing and touching. After being rock-hard for well over half-an-hour, Thomas finally orgasmed after about ten minutes with Steven.

As the weeks went on, we started to talk about our imminent moves to different colleges where we wouldn't be able to meet up so easily.

Samantha was a little older than the rest of us. She'd once been to college, although she dropped out halfway through her course when a well-paid job came up. Her brother Jack, by contrast, was adamant he wanted to own a gym and felt he didn't need qualifications to achieve this, even ones related to fitness. No amount of talking to him would change his point of view, so we ended up leaving him to his delusions.

As September approached, and the other four of us had offers for courses, we started to think about what to do to make our last session special. When we all turned eighteen, we'd generally played out our fantasies, including threesomes and anal sex, and there weren't many new things we could do with each other.

An early suggestion was to hire a private room at a youth hostel or even a small apartment. One night wouldn't have been too expensive between six of us, but it was really no different from doing it at Samantha's place, plus we'd need to be quiet.

Another idea involved inviting our classmates Kath and Michael, who were in our school masturbation group – and were also first cousins of each other. At first, they wouldn't even be naked together, but they'd formed a close bond and had recently become an official couple. Supposedly, their family was furious. After discussing the matter, we decided it was too much of a risk to invite other people at this stage, especially as we'd kept this 'splinter' group a members-only secret all this time.

There were also two other proposals that kept popping up: early one morning, before the crowds, we would find public toilets or to go to the unisex changing at the swimming pool. In both places, we could only fit two or maybe three people into a cubicle so it wouldn't be a true group session, and the toilets posed the additional problem of sneaking the boys into the Ladies or vice-versa without being caught.

Eventually, Steven had the suggestion of going to a forest in the next town. He'd been walking there with his family one weekend and learnt about an abandoned concrete shelter that was probably built during the Second World War. You could easily enter if you didn't mind going way off the path, so not too many tourists did it. We decided to try it out.

Catching the earliest possible bus on a Sunday morning, reckoning that would be the quietest time, we were tired and excited in equal measure. The plan was to have our fun first, but to keep it quick; we would only masturbate and not have sex. In the afternoon, we'd enjoy our juice and sandwiches and wander around a bit. But Stephen's map-reading skills and memory weren't up to scratch, and it took nearly an hour to find the shelter, by which time other visitors were starting to arrive.

There were no seats inside, which meant sitting on the concrete floor, and only one high window, so it was quite dark as well. I wish I'd thought to bring a plastic bag to sit on. However, I'd had the foresight to wear jeans with an elasticated waist so I could easily slip my hand underneath. I'd also brought my long jacket in case I needed to cover up quickly.

Us women had our vibrators handy, but we started off with our fingers in our panties while we gauged the noise level. Similarly, the male half of the group wasted no time in starting, and when my eyes adjusted to the dark, I enjoyed watching their erections poke out as they pleasured themselves.

About ten minutes into the session, we heard the sounds of voices and people walking on leaves and twigs. Each of us covered up straight away. It occurred to me at this point that we hadn't agreed on a 'decoy' activity so it would look less suspicious, although I couldn't think of anything that wouldn't look suspicious in this scenario. The voices seemed to be a couple arguing about which way to go next, but they didn't look in. When they disappeared, we restarted, and I stroked myself again until I was wet.

In spite of our no-sex agreement, the more aroused the group became, the more we talked about getting physical. Tiff and Samantha started to run their hands lightly over each other's boobs, while Jack was stroking Steven and vice-versa. Still masturbating with one hand, I reached out to Thomas with the other and went down on him.

Everyone was half-listening out for anyone else going past, but we were all enjoying the sensation too much, and I personally liked the little slapping noises the others were making. It wasn't long before I heard Tiff trying to stifle her moans and screams. I looked up briefly to see Samantha pounding two fingers in and out of her as if on a mission to get her off. To her credit, Tiff did manage to keep all the noise in – other than one very short scream just as she climaxed.

Meanwhile, keeping an eye on them while still pleasuring Thomas, the boys were in a tandem rhythm that usually meant they were about to spurt. In all my time with the group, I can't recall seeing any two members orgasm exactly together; one was always ahead or behind the other. But I swear their first loads of semen shot out at precisely the same time.

They then scooped up each other's mess and by force of habit, held them out to me. I kept Thomas stimulated with one hand and myself with the other hand, but leaned over to lick up the liquid from the other boys' fingers. Instead of swallowing immediately, I held it in my mouth for a moment as I went back down on Thomas and let him feel the sensation of the semen against his head, and he seemed to like it.

Just then, I heard Samantha starting to orgasm. She wasn't quite as vocal as Tiff and found it a bit easier to stay quiet, but her moans were still distinctive enough to tell she was struggling against it. I was about to bring myself there when we heard another noise outside. I stopped my pleasuring and we all immediately covered up and sat still.

In the distance, a woman squatted down and was peeing in the trees. I found it fascinating; I'd heard others before in neighbouring cubicles, but I'd never seen it for myself. It lasted only a short time but it seemed to take her ages to straighten out her dress. Who wears a dress for a walk in the woods? If she'd turned round, she probably would have seen us before she left, but we were fortunate she didn't.

Once we were sure she was away and nobody else was around, we resumed what we were doing. Thomas had started to go soft and I wasn't as wet, so we stimulated ourselves once more. I went down on him when he was hard again, but the more I did, the more I wanted him inside me. After a few minutes, I came up, but kept stroking him by hand.

'Look, I know what we said,' I whispered to the others, 'but does anyone have a condom?' Thomas seemed to get even harder, somehow, at the mention of this.

The group looked at each other, but all whispered, 'No.'

'Just wondering.'

'You're on the pill,' added Tiff. 'You're protected already.'

'But that's just in case. We could dry hump instead.'

'Why would you do that?' asked Tiff. 'That's boring.'

'But you trib with Samantha.'

'Only if we don't have the strap-on. Just let him inside you. You'd like that, wouldn't you, Thomas?'

'Hang on,' I reminded her, 'this is about both of us.'

Tiff and I discussed the matter in lowered voices for a few more minutes, Thomas occasionally chipping in. We agreed a compromise: Thomas could enter me but he had to pull out before he orgasmed.

I stopped pleasuring him and lay on my back. He wasted no time in moving towards me, then placing his penis head so it touched my vagina.

'You sure about this?'

'Yes,' I said, nodding enthusiastically, 'but remember you need to pull out.'

The others watched as he slipped naked into me. It was a different sort of feeling, a little more friction than usual without the smooth condom. He immediately began his thrusts and it was wonderful. I'd even stopped caring whether there were people outside, although the others split their time between keeping an eye out and watching us.

In possibly the shortest time he'd ever taken, he said, 'I'm gonna...,' then thrusted a couple more times before withdrawing and stroking himself. I lifted my top and patted my stomach. He understood, and directed his spurts onto it. Just as I was about to scoop it up and swallow, Steven leaned in and licked it up.

'Hey,' I said, 'that's mine.'

'You ate ours, so it's only fair.'

I brought Steven in for a kiss and tasted the last drops before he swallowed.

And so ended our last session as school friends. I knew I wanted more of this, especially with Thomas and Steven – and after today, ideally with no more fiddly condoms. As I pulled down my top to cover the remaining sticky mess on my stomach, I began to think about a way to do that.