Paying the kids is a stupid idea.
How about making cleaning the school part of their regular school day? They do that in some countries.
My brothers attended an all male Catholic high school. There were Saturday detentions for things like fighting, chewing gum, talking back... The Brothers had their Saturday work force and weeds were pulled, mulch put down, walls painted, windows washed, bulbs planted.... whatever needed to be done. Believe me, you learned a lesson about smarting off to a teacher if your Saturday was spent painting a wall.