I seem to remember reading about a lock system that was completely inside the door, no keyhole, activated by a knocking or tapping device that is pressed against the door surface. If I remember correctly, it was made by an Israeli company. Cannot find the links now, sorry. And I suspect it would be too high $$ for this kind of purpose anyway.
How about better security for locker room door, maybe recording CCTV on the outside?
--J
[edit] Found one example of the keyless lock. I'm sure there are other keyless lock designs that might be cheaper.
http://www.keylesspro.com/knocklock.htm