‘It’s like the [Education Department’s] motto is, ‘If it’s not broken, break it.’” So said state Assemblyman Jeff Dinowitz, in an apt summary of plans by New York Mayor Bill de Blasio to diminish standards at eight high-performing public high schools.

Mr. Dinowitz, who was quoted in the New York Post, is a proud alum of the Bronx High School of Science. In America’s largest school system, where most children are failing proficiency tests in math and reading, only a modern progressive such as Mr. de Blasio could think the solution is watering down standards at the schools where students are achieving.

The mayor is alarmed because Asian students are disproportionately doing far better than black and Latino kids. At Manhattan’s prestigious Stuyvesant High School, for example, 2.8% of students are Latino and 0.69% black. But 72.9% are Asian-American.

The disproportion is similar at other high-achieving New York City schools where admission is determined by an achievement test. Mr. de Blasio’s solution requires taking seats at these elite schools from Asian or white students and giving them to less qualified black and Latino children who may not be prepared for the academic demands. Either he’s setting these students up to fail, or he’ll have to ruin the schools by dumbing down their standards.

The mayor wants to scrap the Specialized High School Admissions Test and introduce a quota of 20% for students from high-poverty schools. He complains that though there are almost 600 middle schools across the city, “half the students admitted to the specialized high schools last year came from just 21 of those schools.” He’s right that the school system he presides over does a grave disservice to black and Latino children. But he’ll never admit that the reason is because the public schools are run by and for adults, i.e., the teachers unions that are Mr. de Blasio’s political allies.