Questions about American highschool

So next year I’m going on an exchange program to America and I have some question. When would be the best time to go (August to December or January to May is when I can go)? Is highschool hard especially for an exchange student? Are people going to be nice or not care? I can’t think of any other questions but if you could just write what happens in day to day life at school that would be great. Thank you.