Hi Alex, maybe she already knows you are a trans young man? The smile may be because she likes the real you. Next time you see her say she has a pretty blouse or her hair looks nice. Then you can work up to saying she has a nice smile and you noticed it a lot when she looked at you. If she’s in any of your classes or you know some of her teachers, talk about that.
It’s been a LOOONNGG time since I was in the school-crush period, but I do remember how awkward I was, even without strong gender dysphoria (that came later for me). I hope you can connect. It sounds like she wants to get to know you.