I have never had a crush on a teacher in my school life. Most boys fall in love with girls their own age or younger than them. Girls can fall in love with male teachers. Girls tend to choose more mature men. It may be a psychological disorder for female teachers to have a crush on their students. It makes no sense, it should be investigated.
Maybe it is "natural" in some way to have crushes on what is forbidden. But the psychological disorder comes when they look to act out the fantasy.