The San Francisco school board took a small step toward increasing access to the academically elite Lowell High School, allowing all qualified applicants from a predominantly black middle school to automatically earn admission.

The unanimous vote took aim at the racial disparities at Lowell, one of the top public high schools in the country.

The school, which uses primarily grades and test scores to admit students, has a disproportionate number of Asian and white students. Last year, just one black male was among the 500 freshmen, said board member Mark Sanchez.

Lowell generally admits students based on a score that takes into account grade-point average and test results while setting aside a limited number of spots for qualified students from underrepresented schools. The policy revision approved Tuesday lifts the cap for Willie L. Brown Middle School students.

Some supporters estimated that the change would affect about 11 current eighth-grade students at Willie Brown who want to attend Lowell.

The board faced strong opposition from Lowell alumni, who feared the small change to the admission policy would water down the school’s elite reputation.

“We’re not just interested in getting underrepresented students into Lowell,” said Orpheus Crutchfield, co-chairman of the Lowell alumni diversity and equity committee. “We’re interested in getting underrepresented students graduated from Lowell.”

Yet board members angrily responded to what they perceived as rhetoric that implied black and brown students could not succeed at Lowell.

“I’m really heartbroken by the message (the policy) was going to destroy Lowell,” board member Emily Murase said. “This policy is designed to make Lowell stronger.”

Yet the heated debate focused only on the small tweak to the school’s admission policy and failed to address whether Lowell’s overall admission policy — based on academic performance — is illegal.

In recent weeks, some district officials have told The Chronicle that Lowell’s use of grades and test scores to select students appears to violate a state law that bans the use of such factors for admission to regular public schools.

Sanchez has acknowledged that district administrators have raised concerns about the legality for more than a year, yet the district has not acted on the information.

A section of the state’s education code says a selection policy for a high-demand school must have an “unbiased process that prohibits an evaluation of whether a pupil should be enrolled based upon his or her academic or athletic performance.”

While there are exceptions for specialized schools, including performing arts schools, district spokeswoman Gentle Blythe said Lowell is a comprehensive high school.

There are questions, however, as to whether the state code applies to Lowell, since the school near Lake Merced has been using a competitive admission process since the school was founded in 1857.

During Tuesday’s board meeting, Sanchez said that while he supported the change to increase access for Willie Brown students, he believed the board needs to look at the legality of the overall admission process.

Jill Tucker is a San Francisco Chronicle staff writer. Email: jtucker@sfchronicle.com Twitter: @jilltucker