The education secretary has said she is "deeply concerned" about children skipping lessons to attend protests calling for a ceasefire in Gaza.
Pupils were among hundreds of pro-Palestinian protesters at events around the UK on Friday, amid a row over whether they should miss class.
The number of youngsters on strike is unclear but turnout appears fairly low.
Demonstrations took place in various cities,with some signs reading "stop killing children".
Education Secretary Gillian Keegan posted on X, previously Twitter, on Friday afternoon: "I'm deeply concerned that some children are attending political protests during the school day."
She added that "missing school for activism is unacceptable."
Protests listed by the Stop the War coalition included events on Friday in Harrow and Redbridge in London, Manchester, Glasgow, Bristol and Burton, Staffordshire.
Many children, ranging in ages from the very young through to sixth formers, attended with their parents.
Bristolian school pupils handed in a petition calling for a ceasefire to representatives at the city council on Friday morning.
The Green Party's co-leader, Carla Denyer, who is also a local councillor, collected the petition during the event and told the crowd: "[Hamas'] atrocities do not in any way justify the level of bombardment of civilians, including many Gazan children, that has shocked the world."
Hamas gunmen launched an unprecedented assault on Israel from the Gaza Strip on 7 October, killing about 1,200 people and taking more than 200 hostages.
Israel responded with air strikes on Gaza and has launched a ground offensive. More than 12,000 people have been killed, including more than 4,500 children, according to the Hamas-run health ministry.
A pro-Israel demonstration last month in central London called for the safe return of hostages from Gaza, with protesters in Trafalgar Square holding up photographs of those missing.
Among the pro-Palestiniandemonstrators in Burton Upon Trent on Friday were Zubia and her son Yahya.
The 10-year-old said: "I'm here because innocent people are dying. Most of them are children and we need to support them to raise awareness."
Stop the War said it was "providing support" to school students and parents, who it said were "self-organising" the strikes with help from the School Strike For Palestine organisation.
Videos have shown demonstrators in Luton, while students gathered in Tower Hamlets, London, on Thursday in an event Stop the War claims attracted around 400 school children and another 100 adults.
A DfE spokesperson told the BBC: "Children should be in school.
"While we recognise these young people should be able to peacefully express their views, we do not condone them missing out on their education."
The Metropolitan Police was unable to say how many people attended demonstrations in London on Friday.
But a spokeswoman told the BBC: "Strikes and protests by pupils are primarily a matter for school staff, but where they take place it is likely that local officers will be sent to ensure the safety and security of those involved.
"Their priority in these situations is safety but in the event that any offences occur they will respond appropriately."
School strikes are rare - but four years ago, they regularlytook place around the UK and the globe to highlight concerns about global warming. Those protests, partly inspired by Greta Thunberg, have continued but often in lower numbers, after millions of children took part in one event in September 2019.
Additional reporting by David Lumb and Aileen Clarke