There are over 2,000,000 households in the USA which make more than $500,000 a year, and more than 100,000 which make more than $1,000,000 a year.
A family making $500,000 a year really has no problem paying $75,000 a year on highschool for their kid. Moreover, a family making $500,000 a year also usually has savings and other accumulated wealth to draw on.
They simply have to live as though they were a family with above average income, rather than a family in the top 5%.
A family which makes $250,000 is left with around $150,000 after taxes, SS, medical insurance, etc. That means that they need to live off of $75,000 a year. That is around the amount that a family making $150,000 has after taxes, SS, etc.
So, to afford sending a kid to an expensive private high school, and family in the top 5% needs to live as though it was in the top 30%. That means that they need to live off of more money than 70% of the families in the USA live off of.
So “tighten everything” is a relative term.
Of course, if they, for example, also have the $1,000,000 in assets that the top 5% by wealth have, they can probably live better. If they own their house, rather than pay a mortgage, even better.