> People involved with the private high schools liked to brag about the high scores their students scored on standardized tests – without bothering to mention t

> People involved with the private high schools liked to brag about the high scores their students scored on standardized tests – without bothering to mention that you had to score well on such a test to get into them in the first place. This is, as I’ve said before, akin to having a height requirement for your school and then bragging about how tall your student body is. Of course, that's a bad analogy, because average height isn't correlated with the amount of time teachers have to spend on OTHER kids, but average grades of the kids - is (inversely), so parents still prefer schools where other kids are as "smart" as possible, because that indirectly benefits their kids (more attention of teachers for their kid, higher expectations, peer pressure to perform, etc). So - selection bias is one of the reasons kids do better, but that doesn't mean it doesn't also make teachers do better. By the way, in my country (Poland) public universities are considered better, than the private ones (because of historical reasons, and the way free university education works). There's still selection bias (best students go to public universities and don't have to pay for education, so these universities have the best students and teachers, and the best results), but it's mostly unrelated to students' wealth (apart from extremes where kids don't have food or time to do homework in primary school, etc). Do you still consider selection bias in education a bad thing, if it's only about results, not about your family, connections, wealth, etc.? It's easier to teach a group of people on similar skill level, than a group of people where a third is bored, a third is learning, and the rest don't get what's going on.