To start conversation with your crush, you could ask her about something that relates to school.
If you don’t speak English well, just keep it simple.
You could let her know beforehand that you don’t speak English all that well and then proceed to ask her a simple question about school.
It could be about a class, an upcoming school event or even an extracurricular school activity.
Using a topic that relates to the high school will help to make you less nervous when you approach her because you would be using a topic that relates to the both of you.