I'm so sorry this is happening to you. I agree with the other commenters who say report him but use his preferred pronouns in the report to keep yourself safe.

I've been stalked before, more than once. Keep a paper trail of everything. Catalog every time he's tried to contact you, what he said, and on what platform (text, facebook, instagram, etc). The more information you have regarding his behavior the better.

This is not your fault. This is 100% on him and has nothing to do with the fact that you're out. His lesbian fixation is not your responsibility to help him manage, and you did not make yourself a target. He made himself a predator. He's doing this on purpose because he knows he can get away with it.

He also started going around telling people that he's into me at school but that I won't date him because he's trans, even though I'm a lesbian.

That right there tells me he planned on starting a rumor that you're transphobic and essentially forcing you to date him. Truly vile. He's a rapey piece of shit. If someone tries to question you about it just say 'I'm not interested in every woman I see and (she's) just not my type. (She's) being very creepy and aggressive and I'd like for the school to handle this like any other stalking/harrassment case.'