If you don't know the teacher's name, I agree with Mohammed that you should simply say "excuse me" or something similar, since just saying "doctor" does sound very formal. If you know the teacher's last name, though, then you can address him/her in the form "Dr. Smith" (the term "doctor" doesn't seem too formal if you use it with the person's name, since this is the correct form of address for someone with a doctoral degree.) Some university teachers who aren't professors prefer to be addressed by their first names, but you should probably try out "Dr. Smith" first, and let the teacher correct you if he/she would prefer to be a bit more informal.

You would not call him/her 'doctor' because this is far too formal. You would simply say 'Excuse me, I have a question.' Or, just raise your hand to indicate that you want to ask a question or to get his/her attention. Finally, if you know his/her first name, then use that instead. I hope this helps, Mohammad

Dear friends Thank you very much for answer. All the best Mohammad

I was at university twenty years ago, so things might have changed, I was also studying medicine, so that might have made a difference on the use of the title doctor, but we would say "Dr Smith..."