I doubt this would really work because it will say 25 teaching hours. Teachers will know you cannot walk into a classroom without any preparation and post-classroom hours. Even "legally" the teaching hours will just be the classroom hours and somewhere else in the contract it will probably mention "including time to prepare". Even if you put your foot down with the employer, how would you feel walking into a classroom full of kids/adults expecting to learn and you've got nothing for them because you were playing a game with your employer not to work more than 25 hours?