I work in a Secondary School.
I think about 80% of the 'thefts' are actually kids abandoning their stuff wherever it falls and not going back to look for it, as every lesson, there are things left behind and the site team always have at least five things to hand in when they litterpick after lunch.
I find coats, PE kits, bags, sunglasses, prescription glasses, phones, mp3 players, earphones, hats, scarves, gloves, makeup bags, musical instruments, homework for other subjects, pens, pencils, maths sets, calculators, diaries, lunches, shoes, umbrellas, bus passes and on one occasion, a bag containing a wallet with £30, a bus pass, a bank card with the PIN written down and a kid's immigration department ID card.
That last one meant I called his carer, who said he'd been hysterical when he got home because he was convinced it had been stolen during PE and if he'd been stopped by the police, he'd have been arrested and deported immediately. Had he come back to his class, he'd have been handed it within ten minutes of realising he didn't have it. Three lessons after PE, too (so it couldn't have gone missing then, as he'd carried it in).