The road where the accident happened will stay shut overnight.
The Prime Minister said he is being kept up to date after a child died when a school coach carrying children and staff home from an end of term trip slid down a 20ft bank in Somerset.
The coach was travelling back to nearby Minehead Middle School with about 60 to 70 passengers on board.
21 people were taken to hospital, including several who have suffered serious injuries.
The road will stay shut overnight, Avon and Somerset Police said.
Sir Keir Starmer said in a post on X: “A heartbreaking update on the school bus crash in Somerset.
“There are no adequate words to acknowledge the death of a child. All my thoughts are with their parents, family and friends, and all those affected.
“Thank you to the emergency workers who are responding at pace — I’m being kept up to date on this situation.”
An off-duty firefighter travelling behind the coach was able to start freeing passengers immediately, a press conference on Thursday evening heard.
The crash happened on the A396 at Cutcombe Hill, between Wheddon Cross and Timberscombe, at about 3.15pm on Thursday, police said.
Minehead Middle School caters for pupils aged between nine and 14, and is five days away from the end of term.
Gavin Ellis, of Devon and Somerset Fire & Rescue Service, said: “We were mobilised to a major incident of a coach that had overturned onto its roof and slid approximately 20ft down the embankment.
“This was a very complex and technically difficult incident for our crews to deal with, and I’m grateful for the tireless effort and actions of the crews in doing everything they could for those who were trapped and as quickly as safely as possible.”
Emergency services declared a major incident, with 20 double-crewed ambulances, three air ambulances and eight fire engines sent to the scene.
Beacon Education Multi-Academy Trust, which runs the school, said: “It is with deep sadness and heartbreak that we confirm that a pupil from Minehead Middle School has tragically died following a road traffic incident earlier today involving a coach carrying pupils and staff returning from an educational visit.
“A number of other people have been taken to hospital, some of them with serious injuries. This includes two children who were taken to hospital by air ambulance.
“Our thoughts and prayers are with the family, friends, and all those affected by this terrible accident.
“Our entire school community are devastated by this news, and we will do everything we can to support everyone affected.”
A resident who lives near the crash scene described the incident as “horrendous”.
The resident, who asked not to be named, told the PA news agency: “The coach has gone down the slope. It’s pretty steep around here and it’s a big coach.
“There are no barriers along there or anything. We’re used to it, you don’t even notice it.
“I can’t believe it happened, quite honestly, it’s the biggest thing I have known to go over the slope.
“It’s horrendous, and I’m just so sorry for all those kids on that coach.”
A local coach operator, which owns the bus, said it had been liaising with the school and the emergency services.
