This is a great place to start messing with music electronically. We are working to get more resources up on this page in the future!
Audacity - downloadable audio editing
BandLab - has a colorful and user-friendly DAW
Chrome Music Lab - Various music creation modules
Incredibox - Web and app based mixing station
Sound Infusion - DAW that utilizes loops and instruments with a heavy focus on global music perspective
Soundtrap - An online DAW that allows collaboration.
Super Looper - online pad for creating and recording simple loops
This is an incredibly powerful engine that will allow you mix sound at an incredible rate. You will need to create a username, but you can just sign in with your school Google account and you will fly. There is a ton of stuff here so make sure you go through the demos. Have fun producing.
Simply click "Enter Studio" at the top to start!
Christopher Bill has put together this great document that helps guide you through the long process of putting together the great multitrack virtual ensembles that you see on YouTube.