Your school is not bound by the rules of evidence or standards of proof that apply in court: if the school decides that the "evidence"--what other people say about what happened--is credible or believable, the school can discipline you for it.If you suffer a consequences, you could try to sue school for violating your right to education--in court, the rules of evidence, etc. apply, so there would have to be evidence. And/or you could try to sue the person(s) who spread the rumor for defamation. But if there are tapes of her entering and leaving your room, or she sent texts or social media posts referencing this, or if the girl herself, when subpoenaed and testifying under oath (and so subject to punishment if she lies) tells the truth about what happened, your lawsuit(s) will lose.