What ever you do, do not talk to your friends in class or pass notes, like how we do in the US.
Your friends will get reprimanded in class! They are very strict there. Save the talking for between classes. Everyone will want to talk to you b/c you are a teenaged American.
The Spaniards are very friendly people and will ask you lots of questions! It's always nice to bring photos (or even American magazines like People) from 'home' to share with everyone as well.
But remember, the teachers will have all the authority (esp if it is a Catholic school)! Be on your very, very best behavior!
Have a great time!