Can we practice too much? Yeah, I guess, but we can also drink so much water that we die. There is something called "overlearning" that is the key to being able to sit in a musical situation and play our fannies off. Practicing too much becomes an issue after we have achieved (and earned) overlearning, and practicing too much only occurs when the muscles don't have sufficient time to recover between sessions. If you are going to school, you probably don't have enough time in the day to be in danger of practicing too much! Have fun practicing!