If they are my own student, nope. Conflict of interest. If they are of legal age and is not my student in any shape or form, yeah,probably. Might still have problems when they want me to help with their work though...
A crush on a teacher is typical. What's not is acting on those feelings. Don't do it, its unprofessional and actually creepy if the teacher plays along. There was a teacher at my school who simply flirted with her students and she was fired.
We have a PE teacher at our school who's always flirting with the year 11 (age 15/16) boys. She seems to get away with it. I have a crush on my rather glamorous history teacher, so...