What do you usually say in the UK when you come into the class while the teacher is teaching. Is it ok to say "excuse me, sir or miss" or "May I come in, please"? are there any other ways to say this?
In line with the clarifications: I did go to school in the UK (primary through to university) and can confirm the assertion posted by Janus Bahs Jacquet. The teacher has the floor, and it would be seen as very ill-mannered to distract the teacher and class by saying anything at all. You creep in, trying to be as silent and invisible and possible. If the door is closed but unlocked, you don't even knock. The only exception I can think of would be a science practical class, in which there might be safety implications to just walking in without any knowledge of what is going on.
I now teach a module of a Translation Studies course at a Chilean university, and this is something I always go over in the introductory talk as it's a cultural difference.