The BBC report a female teacher has been stabbed to death in front of pupils inside a school in Leeds.
The 61-year-old victim, who the BBC understands was Spanish teacher Anne Maguire, was taken to hospital but later pronounced dead.
A 15-year-old boy was detained by teaching staff at Corpus Christi Catholic College and later arrested.
Police said there was “no ongoing risk to pupils or staff”.
Downing Street said the prime minister’s thoughts were with the victim’s family.
Two pupils at the scene told the BBC that children were seen screaming and running from a classroom following the attack.
A West Yorkshire Police spokesman said Mrs Maguire received a number of stab wounds and a knife was recovered from the scene.
The 15-year-old pupil was detained by other members of teaching staff and arrested in connection with the stabbing, he added.
No-one else was injured in the attack and no other student or member of staff was threatened, the spokesman said.
He said the attack was an “unprecedented event” and schools in Leeds were “generally very safe places” to work and study.
Staff members were “in shock” and Mrs Maguire’s death would “understandably have a massive impact” on her family and the school, he added.
The BBC’s education correspondent Sean Coughlan said the incident would bring back memories of Philip Lawrence, a head teacher who was killed outside a school in Maida Vale, London, in 1995.
Mr Lawrence was murdered when he went to help a pupil who was being attacked.
The call to police in Leeds came from the ambulance service shortly before 12:00 BST.