A rate that 75% can afford without losing sleep does not mean that the other 25% can’t afford it at all.
Let me make up some numbers here. Let’s say it is reasonable to expect a family has 100K income. If you divide the number of children in school by the number of families, you have a rough idea of how many children there are per family (of course, some adjustment for children in multiple schools is needed). Let’s say the average is 6 per family. It might be reasonable to arrive at $5K per child. This would mean that a family with 6 children in school pays $30K. If they earn 100K this is doable without debt. There would be some strain entering the budget for this family if they earned say, 85K or less.
25% of families might have more children or less income or both. They might decide to home school some children rather than find ways to pay, or they might not be able to homeschool. If they decide to send all their kids to school then they will have to apply for scholarship (from voluntarily donated money, like respectable institutions do, not like yeshivos do now), or raise money for the school through dinner campaigns, or trade services to the school, or a thousand other options. Or, they might live in debt. But the point is that 75% have no sweat.