Combination padlocks tend to only lock on side while keyed locks usually have a double-lock shackle. To open a single shackle lock, you create a shim by cutting out a piece of aluminum shaped like the letter M. The middle indentation of the shim is inserted in the space between the lock body and shackle and then the shim is turned while working the shackle up and down until the shim has moved around to the other side of the lock into the locking mechanism. Once the shim is there, use it to push the locking mechanism out of the way and open the lock.
I had a friend in my school days who would ask for empty pop cans to make shims from them. He would then proceed to open a random locker and lock it again because he could. Then again, this is the guy who got away with using a key padlock instead of the required combination lock because it looked almost indistinguishable from the locks that the custodians put on the unassigned lockers.
No, I do not encourage nor endorse illegal actions. This post is just for informational purposes.