I am not surprised at all of this. I attended private schools most of my life except my freshman year in high school (due to a move). I was put in all the advanced classes, yet I recall that most of what was taught I already studied in the fifth or sixth grade in private schools.
That is just the reverse of our family's experience. Our offspring attended private schools K-12, scored high on standardized tests and did very well in college and grad school. We were careful in our choice of schools, which were private but not religious schools, and which acknowledged the place of religion within U.S. history and taught about it respectfully.