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

OMG that is just TERRIBLE!! What I think あなた need to do is first get your boyfriend to trust あなた again. Go to him, look him in the eyes and swear on your life あなた never had sex with that guy. swear あなた never even dated. swear that the only person あなた 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 あなた won't be his friend anymore, say あなた will go and slit your wrist right now または do anything that would emotionally blackmail him. good luck! if あなた need anymore help, feel free to message me! I am happy to help!