Nobody can remember who first suggested masturbating together, but this year it's become our little treat after games class every Wednesday.

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

I'm one of seven girls, and six of us take advantage of this privacy. Maria doesn't join in because she doesn't masturbate, but she doesn't stop the rest of us enjoying ourselves.

We have a few rules: everyone is sworn to secrecy, phones have to be off, we can watch each other but only touch ourselves, and we're not allowed to laugh at each other's techniques or moans. There's also a one-orgasm limit so the teachers don't ask why we took so long, except when 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. 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 have different techniques. Some only touch outside (which I do), some put fingers or a brush handle inside (I wouldn't), and some of them really struggle to keep quiet when they orgasm (I can do it almost silently). The only thing in common is we all touch our nipples in some way.

Another interesting thing is how different we all look 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 anyone get horny before. One thing that's really interesting is my clit doesn't show even when I've got a real 'lady boner' but some of theirs get so big, they stick out a bit like a boy's erection.

Speaking of which, my friend Tiff suggested we invite the boys to join us, saying we fantasise 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 do anything.

Well, the trial was 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. It was the first time I'd ever seen males naked in real life – never mind horny or masturbating – and 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 now all forteen of us just squeeze into ours to change. It's a challenge not to touch each other, especially because there are only eight showers and a lot of erections sticking out. Though some of the girls like to tease the boys by slowly soaping up their breasts and butts 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're now quite competitive with who can orgasm first. So far, it's exactly fifty-fifty.

A few things surprised me about male masturbation. Unlike girls, they do it just one way: a constant up-and-down motion, and they never touch their nipples. Sometimes it's mesmerising to watch seven of them do the same movement and rhythm at the same time. I was surprised to find it takes just as long for them to orgasm as us. I thought they would just get off straight away. They also make no noise when they finish apart from breathing a bit heavier.

The most fascinating bit is I never knew boys always (and I mean always) shoot semen when they orgasm. I thought it was something that sometimes happened and sometimes didn't. They let it go on their bellies and sometimes it goes up to their chests. I love watching that but I wouldn't love to wipe it up every time. Of course, they can only shoot once at a time, so as a treat for turning seventeen, the birthday boy was allowed to orgasm first before the rest of us.

It's a real shame for Thomas who can't get hard enough to orgasm, even though he tries his best. The other boys pick on him but Maria always tells them to stop it.

Unfortunately, it's coming up for the end of the school year so our very last Wank Wednesday will probably be tomorrow.

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

The boys also say they would only do it with girls in the room otherwise it's 'gay'. That's so stupid, especially because there are two who always sit next to each other and have a lot of knowing smiles. I think there's definitely something more going on there.

To celebrate the final time, we've all agreed not to masturbate until next Wednesday, which is really hard for a twice-a-day girl like me, so I’m half-expecting a big puddle of all our juices. We haven't been able to convince Maria to take part, but I hope Thomas manages to orgasm just this once. It's also the deciding day for the cousins: whichever one can finish first will win their competition.

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've 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 won't meet again, masturbation will definitely be something I keep doing with friends and boyfriends, even when I start having sex. I hope the excitement from watching others and being watched never goes away.