what should u do if somthing really bad happends at school?

OMG that is just TERRIBLE!! What I think 당신 need to do is first get your boyfriend to trust 당신 again. Go to him, look him in the eyes and swear on your life 당신 never had sex with that guy. swear 당신 never even dated. swear that the only person 당신 would have sex with is your bf. swear on your life... being emotional and honest at the same time is a great way to gain back trust....
then talk to your so-called friend and ask him why the HELL did he say something like that. Get him to spill out the truth in front of everybody he lied to. threaten him, say 당신 won't be his friend anymore, say 당신 will go and slit your wrist right now 또는 do anything that would emotionally blackmail him. good luck! if 당신 need anymore help, feel free to message me! I am happy to help!