Nobody could remember who first suggested masturbating together, as far as I knew, but that year it became our little treat after games class every Wednesday.

We went to a private school with a massive sports field. It had a large changing block split in two halves by a narrow corridor. Our lockers and showers were to the left, and the boys had their own to the right. The games teachers had a policy of never entering the pupils' changing block except in an emergency. The rumour was that a teacher was once caught spying on the kids, so they didn’t want to be accused again.

I was one of seven girls, and six of us took advantage of this privacy. Maria didn’t join in because she didn’t masturbate, but she didn’t stop the rest of us enjoying ourselves.

We had a few rules: everyone was sworn to secrecy, phones had to be off, we could watch each other but only touch ourselves, and we weren’t allowed to laugh at each other's techniques or moans. There was also a one-orgasm limit so the teachers didn’t ask why we took so long, except when any one of us turned seventeen and the birthday girl was allowed two.

At first, we had this weird thing of showering together as always, but before touching ourselves, we'd dry off, put on our underwear and do it through the material or slip our hands underneath the waistband. But soon, we dropped the clothes and just showed each other our bits. Although I wasn’t attracted to girls, I discovered I loved watching them and I got turned on by knowing people were watching me too.

It was an eye-opener learning that all my classmates had different techniques. Some only touched outside, some put fingers or a brush handle inside, and some of them really struggled to keep quiet when they orgasmed. I could did it almost silently. The only thing in common was we all touched our nipples in some way.

Another interesting thing was how different we all looked down there. I'd seen plenty of other females naked before in changing rooms, but I'd never taken a good look at their genitals and I'd never seen a girl get horny before. One thing that was really interesting was how my clit didn’t show even when I had a real 'lady boner' but some of theirs got so big, they stuck out a bit like a boy's erection.

Speaking of which, my friend Tiff suggested we invite the boys to join us, saying we fantasised about them anyway, so wouldn't it be hot to watch them at the same time? The rest of us immediately said no because what we were already doing was private enough without anyone else being there, never mind the opposite sex. Kath especially didn't want this because one of them was her cousin Michael.

Tiff persisted and the idea grew on us, so we made a compromise: as a trial, we would ask two boys to join us after the next game (not Michael), but if it felt weird or if they did anything out of order, they would be kicked out and we would stay a same-sex group. We also came to an agreement that if we let the rest of the boys in, Kath and Michael would sit back-to-back so they wouldn't see each other doing anything.

Well, the trial was – as I squealed at the time – a-ma-zing! The girls initially did it with clothes on like we used to, but the boys had to take off their underwear to properly stroke themselves, so we soon decided just to let them see our bits. We were soon convinced to let them all join us. Maria agreed, even if she still didn't join in. We gave the boys our rules and they agreed to follow them.

For the first couple of weeks, they changed and showered in their half of the block, then after the game, they could easily sneak across the corridor to our half. They soon started calling it Wank Wednesday.

By the third week, they didn't even bother going into their half and all fourteen of us just squeezed into ours to change. It was a challenge not to touch each other, especially because there were only eight showers and a lot of erections sticking out. Though some of the girls liked to tease the boys by slowly soaping up their boobs and curves right in front of them.

In another development, Kath and Michael had stopped sitting back-to-back as they became more comfortable in each other's presence. In fact, they became quite competitive over who could orgasm first.

It wasn’t the first time I'd ever seen males naked in real life – I’d watched an ex-boyfriend masturbating – but a few things still surprised me. Unlike girls, they did it just one way: a constant up-and-down motion, and they never touched their nipples. Sometimes it was mesmerising to watch seven of them do the same movement and rhythm at the same time. I was surprised to find it could take just as long for them to orgasm as us. I thought they would get off much quicker, like my ex-boyfriend did. They also made no noise when they finished apart from breathing a bit heavier.

The most fascinating bit was I knew boys always (and I mean always) shot semen when they orgasmed. Some let it go on their bellies and sometimes it went up to their chests. I loved watching that but I wouldn't love to wipe it up every time. Of course, they could only shoot once before going soft, so as a treat for turning seventeen, any birthday boy was allowed to orgasm first before the rest of us.

It was a real shame for Thomas who couldn’t get hard enough to orgasm, even though he tried his best. The other boys picked on him but Maria always told them to stop it.

Unfortunately, it was coming up for the end of the school year and that meant our very last Wank Wednesday.

Some of us were old enough to leave school and some were talking about leaving the team, so there was no guarantee how many would be left next year. I asked the others if we could carry on doing it over the summer and into the next school year. Some said they might but nobody said anything for definite, and others wanted to stop after this. Even Tiff said she'd now rather have sex with boys than masturbate with them.

The boys also said they would only did it with girls in the room otherwise it was 'gay'. That was so stupid, especially because there were two who always sat next to each other and had a lot of knowing smiles. I thought there was definitely something more going on there.

To celebrate the final time, we all agreed not to masturbate until the following Wednesday, which was really hard for a twice-a-day girl like me, so I was half-expecting a big puddle of all our juices. We hadn’t been able to convince Maria to take part, but I hoped Thomas would manage to orgasm just that once. It was also the deciding day for the cousins: their victories were exactly fifty-fifty, so whichever one could finish first would win their competition overall.

I felt I really bonded with my classmates through our shared experience. Before this, I would have said that girls and boys shouldn't even be naked together, but they all proved they could behave themselves even when we were naked and turned on, so I'd be happy using mixed-gender changing rooms in the future.

Although our particular group wouldn’t meet again after that last session, I knew masturbation would definitely be something I kept doing with friends and boyfriends, even when I started having sex. I hoped the excitement from watching others and being watched never went away.