Well, maybe you´ll be going to be Volcano Arenal which is still active, you can see the Lava in the night. Costa Rica has lots of volcanos. I mainly wanted to see the Poas, because there is this lake inside..
There are lots of beautiful beaches to visit in Costa Rica, but since you go on a school trip most will be planned ahead I guess. If you want some fun/ sports, do one of those canopy tours into the jungle, swing from tree to tree, its just supergreatfun
Costa Rica has become very touristy. However, the crime rates, particularly by night, are high in some parts of San Jose -ie. around the main bus station where I lived. If you stay with a family, they are going to show you around for sure! If a situation comes up where you´re left alone, you should simply take a taxi to your place.
Costa Rica is full of nature, wildlife, parks.. a wonderful country with friendly people, everything is full of life! You´ll see... la pura vida