Wendy, before you go outside of your school to find lockers, I suggest that you find the longest-serving employee on the maintenance or custodial staff at your school and ask about lockers.
Years ago, when I needed a run of lockers for a play that had scenes in a hallway of a school, I got nowhere when I asked the principal. When I asked our oldest custodian, he knew exactly where several sets had been stored after they were removed from a hallway. A couple of custodians showed up with the lockers a few days later. These were lockers deemed too beat up to be sold for surplus. The custodians had moved them to storage because they saw their potential worth.
If you don't need lockers permanently, your custodians or maintenance people may have a run that they salvaged to use themselves that they might loan to you.
We built a platform with heavy metal casters, and have been using those lockers since. When not being used as scenery, they work as locked storage for fencing rapiers.
Good luck finding what you need!
CJ Breland