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. 
