(Long post, sorry!)
When I was still at school, we used to have Physical Education for 2 hours first thing on a Monday (which we all LOVED, I'm sure you can imagine). Our lockers weren't very big, so most people couldn't fit their PE kits into them and left them either on top of or below the lockers, out of the way. As it was a small school, and had security doors and gates, we thought there wouldn't be an issue.
After our other morning lessons, we come into the locker room to get our lunches, and people begin to notice their PE bags weren't where they left them earlier. I spot mine right on the other side of the room, and open it. My PE kit, which had only been washed the day before, and had been relatively unmarked after our indoor PE, is now CAKED in mud. It honestly looked like someone had dug up handfuls of mud and smeared them over the jogging bottoms. I was furious, and so were the other girls, whose kits had received similar treatment. We went to the head of our year, who quickly worked out that there had only been one other PE class between us leaving our kits and returning for lunch.
That class were dragged in, and quickly the innocent girls pointed out the ones who had entered without kits and upon being sent to find replacements (the school had a few spare kits, but they weren't very nice) had reappeared wearing surprisingly good kits. The PE teacher felt awful - she thought the group had tried to get out of the lesson by hiding their kits, and then gone to fetch them when it didn't work.
Our head of year made every one of the guilty girls confess to which kit she'd stolen. When the girl who took mine walked up to me, she could barely meet my eye. I took out my filthy kit and asked her if she let her own kit get that muddy, or if she'd thought that since it didn't belong to her, she didn't need to treat it with any respect.
The girls who'd stolen the kits were sent home with them that night and letters to their parents informing them of what the girls had done, and if the kit wasn't brought to school sparkling clean the next day, the girl would have a week's detention. My thief came back with a written apology from both her and her mother, addressed to me and mine.