This is a good point. "Private" encompasses a wide variety of experiences and quality. My private experience with my kid is of the somewhat elitist variety. They simply would not accept expelled students from any school. They heavily curate enrollment to create an environment for success. They're building a cooperative community of Families that will reinforce the holistic development of each student. Parental involvement is required and with fairly high expectations.
If you just throw kids together and expect the syllabus/curriculum to prevail, you'll quickly find the Lord of the Flies elements of public school social dynamics come into play and for a child often are more important than academics. At that point, simply being "private" has no advantage.
> At that point, simply being "private" has no advantage.
Based on the experience given above as well as my own experience, I'm going to guess the poster was talking about Catholic parochial schools. Catholic parochial schools were the original public schools in North America and still operate as such today, except obviously no public funding in the United States (they do in some provinces of Canada).
Either way, they'll accept basically anyone. However, they do consistently give better results anyway, even adjusting for income and social status. Actually, the effect is most pronounced in the lowest incomes. The richer you are, the less difference parochial v public makes (of course going to super cushy private schools puts you at an advantage).
If you just throw kids together and expect the syllabus/curriculum to prevail, you'll quickly find the Lord of the Flies elements of public school social dynamics come into play and for a child often are more important than academics. At that point, simply being "private" has no advantage.