I probably get about 4 erections in the middle of every class. its very very annoying, how can i stop it?

Not much to help: This is a common problem in middle puberty or late adolesence and can be very embarrassing. The good new is that it tends to go away on its own eventually. Avoiding any accidental stimulation of the penis would probably help such as wearing looser clothing to prevent direct pressure

Common teen problem: During some of the teen years, a boy's penis can have a mind of its own, spontaneously becoming erect at undesirable times. The proper fit of underwear and jeans can help by not accidentally stimulating the penis. Ignoring the erection (not thinking about it) also helps. Finally, avoid concentrating on (staring at) classmates one finds attractive, as that may cause stimulation in class.