School in Spain is a little bit harder than in the USA, and the methods are also different. Or that is what I was told (I couldn't compare though).
Through the day, you will attend various classess (not different class rooms, but different topics of study), mainly maths, spanish language and/or literature, physics or chemistry, history, english language, religion or ethics, gym and computering. It may have changed a bit, but that is more or less as it used to be. The teacher usually explains something and then make some exercises or solve some examples. Then the students are given some exercises to make during class time or as homework.
There are a couple of breaks, one at 11 o'clok more or less, and one more for having dinner at 13,30. Classes start at 8,30 and end at 4,30 (again, it depends on the specific center).
You will be subject of curiousity by the rest of the students, that's for sure, but you will experience something very very special too. Being with your friends guarantee you will have a very good time
Don't be afraid, be open minded and enjoy every single moment of this experience. I'm sure you won't forget it. I will suggest not to speak during class time in order to avoid teachers regret your invitation to their classess
And tell us upon arrival at home!
Fernando