It's not wrong to just visit them to say Hello and tell them how you are doing. Nothing wrong with that at all. I think most good teachers of either gender would like to know how you are doing. It's not like you're asking them for a date or something. You're fine.
It depends on school policy.
Personally, as a student trying to learn, I was always annoyed when the teacher took class time to visit with former students who dropped by.
At the least, you should respect current students enough to "drop by" when the teacher is not in class. Either during a break period, or before/after school.
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