Gov. Gavin Newsom signed legislation on Monday giving California school districts two years to begin banning or restricting cellphone use during school hours, an initiative that is intended to address rising concerns over social media and children’s mental health.

The Phone-Free Schools Act, which will require schools in the nation’s most populous state to restrict cellphones by July 1, 2026, is the latest statewide effort to curb phone use by children in classrooms. More than a dozen states in a little more than a year already have passed restrictions, including Louisiana, Indiana and Florida.

Calls for school crackdowns have mounted with reports of cyberbullying among adolescents and studies indicating that smartphones, which offer round-the-clock distraction and social media access, have hindered academic instruction and the mental health of children.

In a study in April by the Pew Research Center, some 72 percent of high school teachers and 33 percent of middle school teachers said that cellphone distractions during class were a “major problem.” The surgeon general, Dr. Vivek Murthy, has called for social media platforms to carry warning labels like those on cigarettes and other addictive consumer products. And Jonathan Haidt, a social psychologist who has studied the effects of phone use, has urged schools to strictly limit the use of smartphones.

Last year, nearly three dozen states sued Meta, the parent company of Instagram and Facebook, alleging that the company knowingly sought to addict children to its platforms.

In California, most schools already have cellphone restrictions or are in the process of developing them. Los Angeles Unified, the nation’s second-largest school district, will ban cellphones starting in January and is working out details.

But Mr. Newsom and state lawmakers, who approved the bill on bipartisan votes, wanted to send a message and ensure that all campuses began cracking down on cellphone use.

“We know that excessive smartphone use among youth is linked to increased anxiety, depression, and other mental health issues — but we have the power to intervene,” Mr. Newsom said in a statement. “This new law will help students focus on academics, social development, and the world in front of them, not their screens, when they’re in school.”

Efforts to regulate cellphone use have encountered mixed results in schools and powerful industry pushback, however. A first-in-the-nation children’s privacy law, passed two years ago in California, has been on pause since last year because of court challenges by a tech industry group whose members include Meta and TikTok.

Many parents have argued that cellphones are important for children to have in an age of frequent school shootings and other emergencies. And some educators regard smartphones as a critical tool for learning that should be incorporated into classrooms.

California, the home of Silicon Valley, has almost 1,000 school districts and nearly 6 million public school children.

The measure was opposed by the California School Boards Association, which argued that it was redundant and that it required local decision makers to adopt policies restricting cellphones “even if, through local community meetings, it was found that there was no need.”

Supporters included the California Teachers Association, which called cellphones “a significant distraction in schools and classrooms.”

Mr. Newsom, the father of four school-age children, in June called for a voluntary ban on smartphone use in California public schools. Mr. Newsom also signed a bill on Friday aimed at protecting minors from social media addiction by requiring social media companies to change the default settings on their feeds.

At least 14 states have moved to ban or limit cellphone use in schools in the past 15 months, according to a tracker from Education Week.

But the policies are dramatically different across the country. In Louisiana and a handful of other states, students are explicitly required by law to power off their devices and keep them stashed away through the school day.

Elsewhere, however, local leaders are opting for looser restrictions to give school officials greater flexibility in controlling phone access.

New York City, the nation’s largest school district, for example, abandoned plans for a cellphone ban after the teachers’ and principals’ unions raised concern about its implementation.

Many states and cities have instead encouraged districts to create policies to limit phone use or offered schools incentives to restrict them. And others like Indiana and Minnesota are taking a middle ground, telling districts they must limit cellphone use while leaving the “how” up to them.