How to write English for a teacher I haven't seen for a long time?

First of all, teachers can't call teachers. Teachers in local languages sound like "teachers" and "people who teach things". Would you be happy if someone greeted you and said "Hello teacher"?

Then long time no see, long time no see, this Chinglish has been adopted by foreigners as an idiom, but if you are not sure, you can use is a long time or it is a long time or even it' s a time.

For example, if your teacher's name is John Anderson, you can say: Hi, Mr. John, long time no see, how are you?

Or hello, sir, long time no see, how are you?