A clever and super easy way of getting into someones locker, which I thought of in highschool, and successfully used is, when a person has the locker unlocked and is putting stuff in/taking out they almost always hang the open lock on the locker latch until they are done, then lock it up and leave. 
During this time you stroll by and get their attention away from the locker for even a second and replace their lock with an identical one that you have the combo to.  When they are finished they will take the lock you replaced and lock their locker, walk away, and assume their shit is safe at which point you proceed to unlock your lock, open their locker and do what ever you were planning on doing.