It depends on the nature of the wink. I used to have a teacher who liked to play pranks on his students, and he'd always wink at us as if to say "don't tell". If the teacher is displaying some other outward emotions or appears to be hinting at something more than a teacher-student relationship, then yes ... it would be wrong.
as Colleen has mentioned, it all depends on the message he's giving out. it also depends how old you are and what you mean by "teacher", are you in high school or are you already in college/uni? cause in the uni I go to... it's quite often that you see a several profs and TAs all friendly and flirty with their students.
Not wrong but considering the danger of non-verbal communication being misinterpreted it probably isn't' a good idea.
If he does it at everyone he should be alright but if it's just at certain people not really.
If he's hot - no....... if he's ugly and old - hell yeah ;) :P
its weird to me and creepy