Perhaps I just take a more conservative view of "incidental".
Two considerations that occur to me:
1) Can your task be accomplished without the airplane? (if the answer is 'yes', then the flight may be incidental, but in the above case the answer is 'no' since your task is moving the airplane)
2) What are you getting paid for doing while the airplane moves? (being paid to go visit a client and happening to use an airplane may make the flight 'incidental', but here the only thing of value to the flight school's business you're achieving is moving that airplane).
I don't think it's totally clear cut, but I don't think I'd bet my certificate on the FAA considering a flight school employee on the clock while ferrying flight school airplanes to or from maintenance as being engaged in an activity 'incidental' to the operation of a flight school. ¯\_(ツ)_/¯