Teachers ought to ignore student crushes unless they are becoming a distraction for the class or a danger to the student. In this case, a meeting should be arranged with the parents to discuss the situation.
Nothing. Student crushes are obviously one-sided, and as such to save face on the part of the student and avoid issues with the school, all should be ignored/dissuaded.
Student crushes on teachers must be absolutely prohibited. It creates unnecessary problems in classroom and fosters unhealthy environment for learning.