Nowhere has the debate been more passionate than in the San Mateo Union High School District south of San Francisco. It is a place that epitomizes the socioeconomic divides that have always characterized American education, with the children of tech executives attending class alongside the children of undocumented gardeners and office cleaners.

An April 16 school board meeting to address grading drew more than 500 people. In public comments delivered via Zoom, many parents and students argued that grades are crucial during the college admissions process. One student said grades provide “compensation and incentive for people to work hard.”

Without letter grades, “What motivation do we have to continue working for the end of the school year?” asked another student, who described herself as having “97s in most of my classes.”

After listening for more than two hours, the five board members, slumped wearily in their virtual boxes, debated one another for another 90 minutes. They then voted, 3-2, against the wish of the majority of the speakers, adopting a credit/no credit grading system for the spring semester.

“Our mission is to provide the students with the best education,” said Robert H. Griffin, a board member who voted for credit/no credit, “and not necessarily the highest G.P.A.”

“Every facet of almost everybody’s life has been disrupted,” he continued, noting that long-term inequities — such as lack of access to quality housing, health care and technology — had been magnified during the crisis, making it difficult for some students to make the most of a chaotic transition to learning from home.

Some of the loudest voices in the grading debate are those of affluent public school parents eager to see their offspring rewarded for hard work in tough courses. They note that many private schools are continuing to issue letter grades, which they fear could put their children at a disadvantage if public school districts do not do the same.