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

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