my friends used to call them by their names and surnames. in fact, once, a friend accidently addressed a teacher as XX without adding the mister. it was funny but thank goodness the teacher pretended not to hear anything! anyway, wouldn't this depend on the teacher-student relationship? i.e if i treat this teacher as a friend, and vice versa, i don't see anything wrong with calling them by their names. in fact, my class thought of a english name for my teacher (we were out shopping with her) and irritated her by calling her by tt name the whole day.
