Maintenance time is calculated only of the m/r or equivalent doc period. What time you or not is up to you. This dose not mean you may go against the requirements of any regulations.
Wheels off to wheels on is actual maintenance time per the regulations.
Flight schools are free to charge what ever they like and calculate it how they wish.
Maintenance on the ground ie engines runs are not part of any time that is to be recorded on the m/r ie it dose not form pard of any additional hours to be recorded against tbo time.