It's completely normal. I use to have a crush on one of mine :P You just can't act on it, and they most often go away.
Normal, yes. Good, no.
It's f***ing creepy to have one, but yeah.
Yes when you are in the adolescence stage of development
It's not. Teachers teach, and take no roles in relationships.
What is your definition of normal? "What's normal for the spider is chaos for the fly?"-M. Adams. Is it acceptable or conventional? Heck no. Does it happen anyways? Yeah.