Because teachers are in a position of authority over their students. The law doesnt want a teacher having a sexual relationship, or any romantic relationship at all, with a student.

Is it wrong to have a crush on a 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.