As far as your student is concerned:
The service ceiling is the maximum usable altitude of an aircraft. Specifically, it is the density altitude at which flying in a clean configuration, at the best rate of climbairspeed for that altitude and with all engines operating and producing maximum continuous power, will produce a given rate of climb. A typical value might be 100 ft/min (0.51 m/s) climb,[1] or on the order of 500 ft/min (2.5 m/s) climb for jet aircraft.
Wikipedia says it “might be” and offers no other reference.
Maybe explain to your student that service ceiling is still a relatively “useful” ceiling and the absolute ceiling is the airplane hanging on by its fingernails (aerodynamically speaking) and is nowhere close to optimal altitude ( for its weight) or economic sensible cruise altitudes.
I think your student is getting hung up on a definition rather then understanding the underlying reasons.