> High schools that tend to send high numbers of people to schools with acceptance rates less than 20% tend to be dis-proportionally represented.

It goes further back than that. I went to one of those high schools - certain middle schools are disproportionately represented because those middle schools can prepare students better for the exams. (I didn't attend one of those middle schools, so I can't comment on the presence of feeder elementary schools and feeder preschools, but one can imagine...).