I'd like to make a comment based on this quote from the article:

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She was so bored that she no longer wanted to go to school. So her mother, Lisa Clemans-Cope, spoke to Bradley Hills about providing more challenging work. She was told that her advanced learner would get it. But “what we saw was so sparse and totally inadequate,” she says.

Finally, Clemans-Cope signed up Eleanor for an entrance exam for the Johns Hopkins Center for Talented Youth, Johns Hopkins University’s competitive program for students in grades two through eight with exceptional math and verbal reasoning skills.

Eleanor passed easily, but her parents found the courses too expensive. She now attends weekly math sessions at a private after-school academic enrichment program with her best friend and several neighborhood kids. It costs her parents about $100 a month.

“You have to do something if your kid is not learning,” Clemans-Cope says. “We cannot sit and do nothing while [the school system works] things out.”

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Schools can make sure no one gets ahead in school (although this is wrong), but they end up pushing the ability-grouped, advanced classes in math and other subjects into the private realm. Lower-income and minority students will be under-represented in gifted programs and in "top track" classes, because there is an achievement gap, but the disparity may be even larger in private classes, because parents must have the means, the background knowledge, and the desire to seek them out. We know what AMC, AOPS, CTY, EPGY, SET etc. stand for. I think most of the general public do not.