Ds1's school used to insist that PE kit was stored on open racks outside classrooms, and kit was always going missing - sometimes taken by mistake, because they all used the regulation kit bag, and sometimes taken as a 'joke' - to be hidden either all in one place, or all over the school.
I complained on more than one occasion, when his kit went missing. The first time it turned up - well, ds1 and dh found it, when dh insisted on being allowed to search the school after hours, but the second time, it didn't turn up for nearly a year - by which time we had had to replace it - and like yours, pinkichiban, it was specific school kit, that could only be bought from the uniform shop run by the school - and when the rugby socks alone cost £6, you can tell that the rest of the kit isn't going to be cheap - we had to buy rugby shorts and shirt, indoor PE kit, rugby socks and sports socks, kit bag, two pairs of trainers and a pair of rugby boots - all in all, over £100, even buying the trainers at the cheapie shoe shops.
I told the school I held them responsible, since it was their policy that meant that his kit was not secure, and therefore I thought they should a) reimburse me and b) provide secure storage, but no joy from the school. I am just glad we moved, and the school the dses went to up here has a far more sensible kit policy - basically shorts/tracksuit bottoms and t-shirt/trackie top, any colour, any brand, just no football colours - so it is cheap to buy.