MY IPHONE TURNS ON IN MY LOCKER AND DRAINS Battery

Ok so on the bus ride to school i listen to music on my iphone 4. When i get to school i turn off my iphone, i always double check! I put it in my locker than go to class. After school i come back and my iphone is at freaking 4% and its on! I don't know what is going on!

Could something be pressing the home button? Do you have the phone set to promptly go to sleep? Is the phone receiving messages or other things that wake it up? Lots of possibilities.