It is definitely normal to have crushes on teachers. If youre having the urge to act on your crush or to let your teacher know, then that can be a problem. As you know, a relationship between you and your teacher is not possible. It is, in fact, illegal, for very good reasons.

Is it bad if I have a crush on my teacher?

It is definitely normal to have crushes on teachers. It is normal to have crushes on anyone in high school, but when someone stands in front of a classroom, is confident in their teaching, and is passionate about their subject, it is very understandable that they would be admired.