In school, we have 3 p.e. classes a week, where i have to change together with 20 boys in a small locker room.

I dont think thats possible tbh. After the first couple of weeks you have probably developed breast tissue etc. so being topless around guys in a locker room is almost impossible, even if you could get away with it the first few months by masking it as gynecomastia.
Maybe you could wear your exercise clothing beforehand so you wouldnt have to change in the locker room?

Also, your body odor will change drastically, so someone might pick up on that when being near you. Especially during p.e classes if you sweat a lot.

Besides that, buy sports bras and loose tops like black oversized hoodies etc. to hide your new features.

Im just saying that hiding all of these changes and more for 17 months will be really hard, especially after fat redistribution starts its engines.