Schools in England have been given new guidance intended to stop the use of mobile phones during the school day.
The government said the move was part of a plan to "minimise disruption and improve behaviour in classrooms".
It added that many schools had already banned phones and the change would ensure a consistent approach.
But one school leaders union said the most compulsive use of phones happened out of school and called the change a "non-policy for a non-problem".
The guidance comes almost three years after the government first called for a ban on phones in schools.
Education Secretary Gillian Keegan told BBC Breakfast that the guidance aimed to offer "consistency to reset the social norm that there is no place for mobile phones in our schools all the way through the school day".
She said there was currently a mixed picture on policy, with some schools allowing use of mobiles during break times and others having an outright ban.
Asked if she would go further and bring in legislation, Ms Keegan said: "We would consider what more needed to be done if it [the guidance] did not work".
The guidance sets out a number of examples to illustrate how a phone-free environment could be achieved, including a total ban on phones on schools premises or rules requiring that phones are handed in at the start of the day.
It also says schools could allow pupils to keep possession of their phones but "only on the strict condition that they are never used, seen, or heard" during the day.
The guidance states that school leaders should "develop and implement a policy... which reflects their school's individual contexts and needs".
It adds that schools have a legal duty to ensure pupils' welfare and that stopping phone use during the day is "essential" to ensuring teachers can deliver the curriculum.
Headteachers are also reminded that they are allowed to search pupils for items banned under schools rules and have legal protection from being sued over loss or damage to confiscated items.
The government also cited a survey of secondary school pupils in which 29% of respondents reported phones being used when they were not supposed to be in most or all lessons.
But the Association of School and College Leaders union said it did not expect the new guidance to have any discernible effect.
"Most schools already forbid the use of mobile phones during the school day or allow their use only in limited and stipulated circumstances," said General Secretary Geoff Barton.
