As an employer, I have learned if you fix others mistakes, they keep making the mistake because they aren't motivated to do it right, but if you have them fix their own mistakes, they rarely, if ever, make the same mistake twice. Knowing they will have to spend the time fixing it motivates them to be more careful. Cleaning the school is like that. If they know they will have to clean the mess, they are less likely to make it in the first place.