In private schools, the schools can throw the misbehaving kid out. A parent who is sacrificing to pay tuition is not going to want to have his child misbehave or underperform. Parents who pay extra tend to care more about their children's education than parents who don't. I sent my daughter to a Cuban-run school so she would learn Spanish, and later to a Lutheran school up to the 9th grade. I am pretty sure she got a better education, but she still needed a lot of help in math, since until she got to HS, none of her teachers seemed to know how to teach it. I had to read the book and explain to her what the teacher didn't.