What do i do when a guy touche my a**?

well everyday at school guys touch my a** and im not that type of person that makes someone feel bad but its really bugging me ALOT what do i say to them to make them stop and i wanna be straight up about it because guys are saying that they get whateer they want from me and when they do touch my a**i tell them not to do it and i get really mad but that doesnt work? any other idea's and dont tell em to tell an adult i dont wanna snitch and dont tell me thats not snitching...