It’s non-linear for most of those things though - a class of 20 kids or a class of 30 kids still need a teacher (although there’s evidence that the former’s going to have better outcomes), books aren’t bought new every year so are amortised over a whole load of students, lunches are cheaper when you can buy ingredients in bulk etc.

Private schools are usually more expensive because there’s a lower ratio of students to teachers. If all the rich kids left, you’d have schools being merged together to save money; you wouldn’t have the few(er) poor kids left behind with one teacher covering multiple classes.