How to Stop Sweating?

i am a pre-teen girl. even when i am sitting still i SWEAT all the time and during gym and healthy activity (recess) i want to take off my coat (which i put on when i find those annoying sweat stains) off but i'm scared that the other kids will see my sweat stains and i don't want the boys to think i'm gross. my school has a uniform so my shirt must stay tucked in (meaning no "wear a baggy shirt" advice- can't do that) THANKS IN ADVANCE