With respect, a dipstick is essential, the gauges cannot be trusted. The planning figure we use is 36 l/h for a C172. You might like to suggest that the school do a couple of short flights with a known fuel load and dipstick to confirm fuel loss is real and not finger trouble before taking matters further.
Early C172's (down to N?) have steel internal tanks, the new ones (with fifty million drains) has a wet wing, I think, and not a bladder. If it was a bladder, a clogged vent line has been known to partially collapse the bladder, resulting in a "full" indication when in fact very little fuel has been added.
Unless the engine is gulping fuel (which I doubt) your options are a bung cap, bung drain, bung tank or bung plumbing (including gascolator drain and fuel pump diaphrams). I think I understand that a bung cap seal would produce an undetectable major loss of fuel in flight (sucked out and vapourised with no stain).
As a total newbie, and being used to the "brace and suspenders" approach of Boeing commercial jets, I was amazed at GA engineering both pleasantly and not so pleasantly, having said that, I found the safety margins in certain military aircraft bits to be simply scary. I guess its the devil you know...
Good luck and please let us know how you go.