You are going to know everybody and everybody will know you. They are probably all very close knit and good friends with each other so I recommend being nice.
Everybody grew up together in that school so the atmosphere will be really, really, really different.
Everyone knows you, and it's usually an honest kind of friendliness. Just don't dress as a scene kid and you will be fine. Also, class content material is usually a lot harder, I recommend not taking AP classes at a smaller/country school.
One's small, one's not. Only difference.