Garageband is a great tool for audio editing and composition.  It's especially easy to create music using "loops;" short pieces of music with one instrument that repeat really neatly.  Check out how to use it here:

Try to create something that captures the energy of the school.  For you, that might mean the playground at lunchtime, or the quiet of the library, or the buzz of students working in class.  Express yourself!

Oh, and in case you feel like expressing yourself in Autotune: