Oh that is kind of funny :) Well it's a good exercise to do! However you can always change it up by saying ''I'm tired'' or ''not so good'' instead of fine. In America we always say we are fine even when we are not, it is a thing that people just seem to say because it's a habit like you said. So we try to challenge ourselves to be more real and open with people and tell them how we actually feel instead of just saying "I'm fine."
And I will say, if I may, that it's actually thank you or thanks, however people will understand what you are saying so it doesn't really matter :) (sorry if that's rude?)
In America we actually don't stand up when the teacher comes in, but that is very respectful of you guys to do!
How long have you been taking English? It's quite good actually. I know it's hard to learn another language so it's awesome that you stuck with it and can use it now. The thai language written is so pretty to me. I'm sorry if I said anything that you didn't understand.