This one girl likes me at school and she kissed me why? She wants to have sex with me and im a girl

Well maybe she wants to do this to you because she might think you are attractive. And maybe she has strong feelings for you.

If you're not interested in girls, tell her.

ok you are both girls,and you say she kissed you ,and went more. the Reason she's lesbian and Attracted to you.but if you hate that just tell her the truth and to not break her heard

You make it sound like such a bad thing. It just means she has feelings for you, like a boy perhaps would. If you aren't interested in her that way, tactfully tell her.