Boston Latin School's interim head master on Tuesday attributed the school's downgrade this year to a "technicality," but state Elementary and Secondary Education Commissioner Mitchell Chester said the 95 percent participation rate in student assessment exams, which the school failed in one subgroup, has been a requirement for 15 years.

The school was downgraded Monday to a level two - the second best rating in the state - because of insufficient participation by eighth grade white students in standardized tests, according to the school.

Founded close to four centuries ago in 1635, Boston Latin School has been accused of failing to adequately address racism against black students, and on Monday U.S. Attorney Carmen Ortiz announced an agreement with the school, while accusing educators there of a "troubling failure" in the lack of response to an incident where a male student used a racial slur against a black female student and allegedly threatened to lynch her with an electrical cord.

An exam school that prides itself on its rigor and performance, the public school, grades 7 through 12, was knocked down to a level two.

"It's really unfortunate especially for one of our best schools, which has a lot of issues right now," Board of Elementary and Secondary Education Chairman Paul Sagan said Tuesday, calling the downgrade an "emotional hit."

According to the Department of Elementary and Secondary Education, a total of 40 white students did not take the math test, including 13 who opted out.

In a letter posted to the school's website, Interim Head Master Michael Contompasis and Interim Associate Head Master Jerry Howland blamed the downgrade on a "technicality" and touted high performance on tests overall. The school administrators said 13 white parents, relying on guidance from the Department of Elementary and Secondary Education, opted out of exams, resulting in white participation dropping below the threshold to 94 percent in English and 93 percent in math.

The letter says the school attempted to appeal the department's decision. According to a department spokeswoman the threshold is for the school as a whole, including subgroups, rather than a particular grade.

"We try to be very clear with districts about the participation requirement," Chester said at Tuesday's meeting. Responding to concerns raised by board member Katherine Craven, Chester said, "You're not getting the whole story apparently from that letter."

Chester said the participation rate, including in subgroups, was a requirement of the federal No Child Left Behind law passed about 15 years ago, and is intended to prevent schools from cherry picking the students whose academic performance is assessed.

There were approximately 40 schools around the state that dropped a level because of their participation rates, Chester said.

The Boston Latin administrators said the parents who opted out of the Partnership for Assessment of Readiness for College and Careers test last spring "relied" on a letter Chester sent out last January that advises "principals and test proctors to handle refusals with sensitivity. Students should not be pressured, nor should they be punished for not taking the test."

To graduate high school, Massachusetts public school students are required to achieve certain scores on standardized tests in English, math and science, which are given their sophomore year.

In a statement released Monday after Ortiz announced the conclusion of an investigation into Boston Latin, the Lawyers' Committee for Civil Rights and Economic Justice, the Boston Branch of the NAACP, the ACLU of Massachusetts, and Massachusetts Advocates for Children said the would monitor Boston's compliance with a resolution agreement.

"We have always asserted that Latin is not the only school in Greater Boston or the Commonwealth to have issues with racial isolation and racial hostility," officials from the four groups wrote in a joint statement. "However, the very public discussion of harassment at BLS has raised significant awareness of these issues across Boston's public, charter, and private schools. And the U.S. Attorney's findings should put every school system in the Commonwealth on notice of their federal duty to address racial harassment."

Boston Mayor Martin Walsh on Monday morning blasted what he called a "flawed system" that downgraded Boston Latin and the Roger Clap Innovation School from Level 1 to Level 2 because not enough students had taken the PARCC exam in 2016.

Walsh, during an interview on Boston Herald Radio, said two students at the Clap School and 13 at Boston Latin had opted out of the test.

"It doesn't mean the education in Latin School and Clap has gone down by any stretch of the imagination," Walsh said. "All it simply means is that parents were given the option to opt out. So for all intents and purposes, parents and students aren't penalized, nor should they be, but the school's being penalized because their parents, or the students, opted out of the test. Completely wrong. It's not right."

Saying he had not had a chance to review the resolution announced by Ortiz, Gov. Charlie Baker told reporters Monday afternoon that the change in accountability levels "doesn't change anything about the fact that Boston Latin is a great school."

"I think Boston Latin is, continues to be, one of the best public high schools in the United States of America, and I think that's a commonly held and appropriately held view by people both in the public at large and the education community generally. The fact that DESE gave parents opportunity to opt out of taking the PARCC exam meant that the number of kids who needed to take the exam for Boston Latin to be qualified as a Level 1 school didn't get met, from my point of view, doesn't change anything about the fact that Boston Latin is a great school," Baker said.

Katie Lannan and Michael Norton contributed reporting.