It's a ripp off!
It's a way of getting more money from you because you are no doubt paying on "chock to chock" time or the dredded Hobbs metre and so the clock is running all the time.
The aircraft/engine time is flying time only so you are paying for the flying and the only cost that "school" is incuring when you are taxing is the fuel that you are burning.
I think that I can guess who would pull a stunt like this!