• Would it be possible for you to get a lawyer (solicitor)? It might help to have someone who can advise you on your legal rights here, and represent your interests. The prospect of a lawsuit could "persuade" your school to do the right thing. I realize that you're only 17; maybe this is something that your family could help you with?
• Others have already suggested that, rather than trying to deal with this through your school, you might want to go directly to the police. All I would add here is to keep in mind that the police are law enforcement; if no actual laws are involved-- it's not a matter of a crime or misdemeanor-- they can't really do anything; that's where getting your school to address it might be the better option (they have no such restrictions).
• Related to both of the above: you might want to look into whether there are any laws against "stalking" where you live, and, if so, how this is defined. Sounds to me like this man's actions may well qualify.
• As per Gavin de Becker's book The Gift of Fear (which, along with other commenters, I'd recommend that you read, if you haven't already), don't respond to this man directly again, in any way whatsoever. Not that I think you WANT to, god knows-- of course your wish has always been to have nothing to do with him-- but just in the sense of telling him "no" and to leave you alone. De Becker emphasizes that you must only say these things ONCE, clearly and firmly (as you have done); repeating them counts as an interaction, which is exactly what he wants from you, and will only encourage him. I realize that he's making it as difficult as possible for you to stick to this; that's what stalkers do. Perhaps you could turn this to your advantage by citing de Becker to your school, and explaining that you can't keep telling this guy to back off (since that's guaranteed to have the opposite effect), so THEY have to do it. (Plus, if they don't, you'll have no choice but to get the police involved, and/or take legal action...)
• Have you considered getting a restraining order against him? Don't know whether that's a possibility, but seems like it could be appropriate in this situation.
• This one is a long shot, obviously, but: you might see if you could tap into some self-policing. Like, put it out there that a "transbian" is behaving in just the sort of way that plays right into TERF hands! Oh noes! "She"'s personally lending credence to all those "cotton ceiling" accusations, and giving "transwomen" a bad name! Making it look as though "this never happens" is a LIE! Sabotaging/betraying the entire movement! Encouraging "transphobia"! For the good of all trans people, a traitor like THAT really needs to be called out...
• Could you get any of your friends to intervene? I know that you said nobody's taking him seriously, but maybe if you told them what a bad effect this is having on you, and how experts (like de Becker) recommend handling it, they might come around? Being able to convey your "no" indirectly (so this, rather than more contact with you, is all he ever gets) might really help. As would other people (like them) putting pressure on him-- via public disapproval of his behavior-- to get lost.
• Finally, please don't forget to look after yourself, in terms of getting the emotional support that you (understandably) need. Perhaps counseling might be a good idea? Someone to help you manage the stress that you're under thanks to this creep? Maybe work with you on strategies for dealing with the situation?

And, just as a reminder: none of this is your fault. You've done nothing wrong by coming out. And you've handled a very difficult (and very unfair) situation in a far more mature, responsible fashion than any of the adults around you. You haven't got anything to be ashamed of. Only he, and the people enabling him (by supporting this misogynistic, homophobic genderist garbage, and/or looking the other way), do.

Keep us posted, OK? We're rooting for you :)