We don't do that in class in China!

What is the biggest culture change I met in the US? Food, or something else? It’s not for me! I am sure I will get used to the American lifestyle sooner. But, now I feel I am encounted with more difficult things which I have never had before and I think I should overcome and adjust to as soon as possible. What’s that? It’s the way to communicate with others, in other words, what should I do when I am in class with the professors and the classmates? Is there any difference between the way to handle the relationship with the classmates and that with the professors?
I feel confused with that, especially when I am taking class with other American students. It seems that I am so shy and less confident among them, not to say playing jokes with the students and professors! It’s impossible for me to do! In China, unless you are familiar with your professors quite well, you are able to do that in class, or it will be regarded as not respecting your teachers and maybe you will get a low mark! So the problem comes. I do not know what I should do and whether I do in a right way. I never expect to offend my professors and the classmates, so most of the time I keep in silence. But that seems not the right way to solve the problem. It’s so different and I think it harsher for me to adjust to the American way to communicate with others, but I do not want to be considered weird or unfriendly.
I need time and understanding.