I was teaching 11/12 year olds at the tim, and all five classes (150ish kids) would gather after lunch on Friday and all would go with various staff to do various sports. All knew beforehand which sport and teacher they were to go with, so we'd just wait until everyone was there and break into the groups.
I had 4 girls in my class who were the classic 'friends one day, mortal enemies the next' group. They decided that they didn't want to participate in Friday sport, so instead of coming to the sport, they would head to the bathrooms and spend the next 2 hours there. If they were caught, it was easy to behave like they'd just been to the bathroom, head out towards the sports fields, and double back to the bathroom again.
Spending two hours in a toilet notwithstanding, they managed this routine for a couple of weeks until the group had yet another falling out and one of the girls came on Friday morning an warned me that the others would be doing this in the afternoon in order to get them into trouble.
So come 1/2 hour into sport, I headed into the bathroom and caught the group of three, and had quite a chat. Unfortunately for the girl who'd dobbed them, they were only too willing to pull her down with them, and informed me that she'd been doing this with them for the past three weeks.
So lesson learned: don't fight with your co-cospirators, and if you do, don't dob them in for something that will get you into trouble as well.