do you think she'd take it positively if you talked to her about your issue before she gets a chance to mess it up? i mean like, do you think she could be pro-trans and help you keep the secret? even if not, if you pass well enough and she suddenly turns up and calls you by a female name, it'd be her who will look weird, not you. still, we can't say what her reaction would be without knowing anything about her. like, she might find this confusing and ask you in private, or she'll scream aloud in public asking what happened to your gender. telling her beforehand might be dangerous as well, if she decides to out you to the rest of the school. think about her personality and think how she'd react. make a good decision. good luck!