When I first heard complaints that the admissions process at Thomas Jefferson High School for Science and Technology had gone soft, I guessed that the cause of the unrest was an effort by our nation’s most selective school to become more diverse.

Of the freshmen who entered the Fairfax County public school in 2010, there were only 13 Hispanic and four black students. That amounted to 3.5 percent of the class, even though 52 Hispanic and 29 black applicants had academic records good enough to survive the first cut. I assumed that the school, reacting to those numbers, had begun to admit more Hispanics and blacks, which in turn might be the source of criticism for a downturn in this year’s freshman grades.

I was wrong. More freshmen this year needed remediation, but growing ethnic diversity cannot be the reason. This year’s freshman class, whose Algebra 2/Trigonometry grades dropped, had 13 Hispanics, just like its predecessor. Next year’s freshman class also will have 13 Hispanics. There were six black students among this year’s freshmen. There will be seven next fall. Bottom line: The black-Hispanic makeup will be a miniscule 4.2 percent.

Many more black and Hispanic applicants had scores and grades indicating that they are capable of doing Jefferson work. Teachers told me of at least one black applicant who was rejected despite being brilliant, clearly capable of handling Algebra 2/Trigonometry. But Fairfax officials have made no moves of late to alter the admissions formula.

I believe Jefferson is the most selective high school in the country, based on its 2011 average SAT score of 2218. I have been unable to find a score that high in any school, public or private, where a majority of students take the SAT.

The magnet school serves a broad area of Northern Virginia. It is supposed to select students based on academic and personal potential in science and math. But so is the Massachusetts Institute of Technology, where the share of black and Hispanic students is 20 percent.

What happened this year in ninth grade at Jefferson? About 30 percent of the students are getting extra help in math and science, according to school officials, because their grades indicated that they were in academic jeopardy or on the verge it.

At midyear, seven Jefferson math teachers said in a letter to the county School Board chairman that the freshman class averages in Algebra 2/Trigonometry were only 81 to 87 percent, below the usual 89 to 92 percent average of previous Jefferson ninth grades. A record number of students had grade point averages below 80 percent.

The students in trouble are almost all Asian or white, the two largest ethnic groups at Jefferson. The school is right to do what it is doing: giving them extra time to learn with after-school help and a planned summer boot camp for next year’s incoming freshmen, as my colleague Emma Brown reported. School officials said the problem may be a rise in the number of freshmen taking advanced math courses not usually given to ninth-graders, rather than a weak admissions system.

Jefferson Principal Evan Glazer told me his goal is for “students to be in an environment with adequate challenge based on their readiness so they can prosper.”

The Jefferson math teachers suggested bringing more good teaching to more students. They said the standards for math instruction in all Fairfax middle schools should be raised. They want the Jefferson admissions process to give more weight to the recommendations of middle school teachers. That could strengthen the importance of personal factors such as motivation, persistence and family background.

That is the way students are admitted to the University of Virginia, another selective public institution. Its undergraduate population is 12.4 percent black and Hispanic. It is not so hard to create an educational institution that is both academically demanding and ethnically diverse if you listen to teachers and believe in the power of their work.