Ask any parent about the time their kids spend on mobile devices, and you’ll likely hear the same refrain: It’s too much.

Excessive use of smartphones and social media is linked to rising rates of teenage depression and self-harm, while also damaging students’ academic performance and exacerbating achievement gaps. At this point, the question isn’t whether phones should be banned from classrooms, but why more schools haven’t done so already.

Evidence about the negative effects of mobile devices on learning is overwhelming. Large-scale international assessments have shown that anything beyond limited use of technology in the classroom harms academic performance. A 14-country study cited in a U.N. report this year found that merely being in proximity to smartphones disrupted learning for all ages, from preschool to college, with poorly performing students suffering the most.

Prompted by findings like these — and common sense — the British government announced this month that it will instruct schools to prohibit the use of mobile phones during the school day. Other European countries, including the Netherlands and France, have imposed similar bans. Such policies can be challenging to enforce, but in places that have followed through the gains have been striking. Bans on phones in two regions in Spain improved math test scores by the equivalent of more than half a year’s learning.

A 2022 analysis of more than 100 Norwegian middle schools found that banning phones boosted students’ grades and test scores and increased their likelihood of attending an advanced high school. It also yielded bigger academic improvements than far more expensive policies, such as reducing class sizes or putting more computers in schools.

Despite these clear benefits, U.S. schools seem to be moving in the wrong direction. As of 2020, 76% of public schools said that they prohibited the “non-academic” use of phones during school hours, down from more than 90% a decade earlier. By all indications, those restrictions are widely flouted. In response to a surge in smartphone use during the pandemic — fueled partly by misguided school closures — some districts appear to have abandoned even token efforts to keep devices out of kids’ hands. A survey released last month found that 97% of U.S. adolescents say they use their devices during the school day, for a median of 43 minutes — with most of that time spent on social media, YouTube and video games.

Arresting this trend is critical to helping students recover lost ground and avoid permanent blight to their careers and life prospects. European-style national bans would be unworkable in the U.S., where schools are controlled locally. But policy makers should emphasize the urgency of the issue. State legislatures should press schools to ban the use of phones for the duration of the school day, including during passing periods and recesses — and to confiscate them, if necessary. They should provide incentives to districts that demonstrate academic gains after imposing school-wide bans. They should also help schools pay for things like electronics-storage pouches and phone lockers.

Schools will no doubt get an earful from parents who oppose such bans. While acknowledging legitimate anxieties — such as how to reach a student during a crisis — they should hold firm and explain that emergency-contact protocols are more than sufficient.

It’s by now incontrovertible that, however essential to modern life, smartphones have no place in the classroom. The sooner schools remove them, the better off students will be.

— The Bloomberg Opinion editorial board