Pupils will not be allowed to have their phones on them at all during the school day under stricter government guidance in England.
Statutory guidance came into force in June, making schools and trusts legally responsible for being phone-free throughout the day.
Ministers now say changes to that guidance will go beyond the current "no see, no hear" policies in place in some schools - where phones can remain in pupils' bags if they are not seen or heard - when it comes into force from September 2027.
But head teachers' unions said schools would struggle to implement stricter new policies without additional funding from government.
Education Secretary Lucy Powell said the government's "clear message" to parents and pupils was that "school is for learning and friendship, not scrolling".
Access to phones "risks pulling children away from their learning and from each other", the government said.
Schools will be able to decide how to follow the guidance. Some schools have invested in sealed pouches or secure lockers. Others collect phones at the start of the day, or have prohibited them from sites altogether.
Paul Whiteman, general secretary of the NAHT school leaders union, said phones could raise "safety issues" and be an "unhelpful distraction" during the school day. But he said the more robust measures recommended under the new guidance would "come at a cost".
Pepe Di'Iasio, general secretary of the Association of School and College Leaders, said the latest guidance "does at least provide some clarity", but added that school leaders "cannot simply make mobile phones disappear".
"If all schools are going to be expected to have lockable pouches or other secure storage facilities for every pupil, the cost involved will be very significant indeed," he said.
"This is not feasible unless the government puts in the funding required to meet these costs."
Dame Rachel de Souza, Children's Commissioner for England, said she welcomed strong action to stop phones distracting pupils and disrupting the school day.
Charlotte Ashton, co-founder of campaign group of Generation Focus, similarly welcomed the changes, saying "no see, no hear" policies were not working and pupils were still "TikToking in the toilets".
"Ultimately, we need to get to a point where it's not normal for every single child to have a personal internet-enabled device in their pockets with them 24/7," she said.
Shadow Education Secretary Laura Trott said she was "delighted that Labour have finally listened to us and agreed to properly ban smartphones in schools".
She said it was also the "right thing to do" to have a review of safety standards around the technology used in schools, which the government has also announced on the back of calls for improved guidance.
But she criticised what she called a lack of "joined up thinking" over what new tech was being allowed in classrooms.
Because education is devolved, the approach to phones in schools varies across the UK.
In Scotland, external, the government has said it will introduce a new law to restrict the personal use of phones in schools, and launched a consultation on the issue over the summer.
In Wales, external, the Senedd recently voted against a motion to introduce a national statutory ban on phones in classrooms. Schools are still encouraged to restrict the use of phones during the school day, but heads have flexibility over how to implement their own rules.
And in Northern Ireland, Education Minister Paul Givan launched a 12-week consultation in August on whether to ban mobile phones from schools by law, calling for "a stronger and more consistent approach across our education system".
Rules around phones in schools are separate from the UK's upcoming ban on social media for under-16s, due to be introduced in March.
Children will be barred from accessing platforms like Instagram, Snapchat and TikTok. Vinted and eBay could also be included in the list of platforms set to be banned, the culture secretary has said.
Additional reporting by Beth Rose and Nathan Standley
