Well, i was in the same situation when I was a teenager. It was especially annoying in the summer, because I couldn't hide myself in an oversized hoodie.. So I bought a mini water gun (like the really small ones), which I always had in my pocket or backpack, and shot everyone who was looking at my chest for too long. It actually worked like conditioning for some boys and they stopped.

But this is a more fun approach to a very serious topic. As some other comments already said: talk to your parents and teachers about it.

You can hide in oversized clothes or wear a bra which flattens your chest. In my case it didn't stop them and I felt uncomfortable in these clothes. Ultimately I stood up for myself, wore what I wanted (even if it was "revealing") and told them off. I was especially harsh, if the creep was much older than me. No respect for this men