But they still get your money, so they don't really care. If every parent pulled their kids from school, the administrators would be happy as clams.
Because of the teachers unions the school will not be able to get rid of tenured teachers. So if 1/3 of the students leave, they lose 1/3 of their payroll but will only be able to get rid of the newest teachers who get paid half of what the experienced teachers get paid. So it would be catastrophic to the school.
Also, these are "school systems" so they can absorb one school losing students, but if 1/3 of elementary school kids throughout the city were to be pulled they would have a significant number of elementary school teachers they couldn't fire, on the payroll, but who they had no jobs for.
Finally, if a third of the kids are pulled then you will have a very large portion of the population motivated to demand the money paid for kids education follow the kid (some states already do this). Imagine 10 elementary school kids begin homeschooling using the local church building. You have parents, Pastors and church staff working together but now you also have $140,000 coming to this homeschool. You could hire 6 retired members and teachers at $19,000 a year (the maximum you can make and still collect your retirement benefits). They could each work part time. You could also spend about $20,000 to the church to help support the staff.
Parents buy food but the people you hire could prepare breakfast and lunch. You can get books from the local library.