How to get rid friends that make fun of u all the time?

i have these 3 friends at my table in art i dont consider them friends but they always just make fun of me when theyre bored. like they send nasty letters to the teacher saying its from me and i just sick of it. i was about to cry today in 3rd and they like always saying just kidding at the end of the day but i really hate them. i would say somthing but there 3 of them and theyll gang up on me and getting adults into it will make it worse. what do u think i can do to make it stop