Hacker News Reader

This says that the average private highschool tuition is $15,645.

In 2003, the average private highschool tuition was $8,412 (Snyder, Dillow, & Hoffman, 2008), or $14,079 (in today's dollars).

Please correct me if I've made some mistake, but plugging those numbers in shows that the annualized real increase in price is 0.5% per year over 20 years.

The first bullet says: “$23,839 is the average annual tuition among the nation’s private K-12 schools.”

What is the rate of increase, and how does it compare to CPI inflation?

There is a chart at the bottom. Costs doubled over twenty years since 1999, directly comparable to the college rate over the same period and well above inflation.

This said nothing about the number of seats in private schools versus public ones. If there are very few then we should expect private costs to increase dramatically as public costs increase. Not every student gets public loans, some students come from wealthy families that have other means of financing. If public is $100k, then these families would still likely choose the $120k private school for better outcomes. There is no reason for private to stay cheap.