Bullied coz of flat chest?

at luch everyday the big gang of bad boys force me to *** to their corner ( its right at the bak of the school it used to be a garbage area but it was too far so no1 goes there not even teachers) it has a big gate and they make me take my top off and vest i dnt wear a bra as i dnt fit in 1 then they get their big rope n make me jump up on the fence and tie my arms and legs to it real tight. they then go and have lunch and leave me their hanging naked in the cold wether. they *** bak after about half an hour and make fun of me and say im a guy and shud start acting lyk 1. they sumtimes get a rope and give me a wedgie with it. i cant tell any1 coz they know my BIG BIG BIG secret!!!! n said they will tell everyone if i gred on them. i never did nothin 2 them im the sort of girl who always is in the library reading books or doing extra homework. tell me wat to do n y they do it plz