What is the Japanese word for teacher?

The Japanese Word for "teacher" is "sensei".

Now, if you were referring to someone who is a peer, like a student in a higher grade than you who teaches you things, one could call them "sempai".

"Sensei" is formal and "sempai" is not formal. "Sempai" best kept out of the class room unless the teacher allows everyone to call them that, and even then, it is too comfortable a name to possibly be inappropriate.