I rang the school and got to chat to head of year (7) She was very nice, said she'll talk to PE teacher and see if they can be more vigilant. I'm pretty sure it's someone in his year as his friend had a lot of cash stolen on their trip to Wales.
Was quite impressed with the school really, they've even said they'll reimburse us.
I'll have to hatch some plan to catch the thief. Was even thinking of a mouse trap but knowing ds1 he'll forget it's there