Children have witnessed a supply teacher being stabbed in the stomach with a kitchen knife as he began to teach a science lesson
The stabbing of Vincent Uzomah, 50, at the Dixons Kings Academy, in Bradford, sparked a police hunt for a 14-year-old boy who fled the school after scaling a fence.
The teenager, who is a pupil at the academy, was arrested in the centre of the city about six hours later on suspicion of attempting to murder the teacher, who had only been working at the school for a few weeks.
Mr Uzomah was taken to hospital with a stab wound to his stomach but is said to be stable and his condition is not thought to be life threatening. Police said his family is with him.
It is understood he was stabbed with a kitchen knife the boy smuggled into the school this morning.
Detective Superintendent Simon Atkinson, of West Yorkshire Police said specially trained staff were talking to the children who "potentially witnessed something horrendous".
Parents who gathered outside the school after hearing the news said the school had a good reputation for discipline but were concerned about the effects on their children.
Police were alerted by paramedics who were called to the school at around 8.55am.
Mr Atkinson said: "There were a number of pupils who witnessed this incident, which took place in a classroom, and they are currently liaising with our specially trained officers who are obviously supporting them at this difficult time.
"This is believed to be an isolated incident and the police are continuing inquiries to establish the facts.
"I would like to reassure residents and staff that all necessary resources are being deployed to investigate this ongoing and clearly serious incident."
Mr Atkinson was speaking in front of the school, which is about a mile and a half (2.4km) from the centre of Bradford, shortly after hundreds of pupils left for the day.
"We're speaking with the pupils there to see what they actually saw," Mr Atkinson said.
"We've got a welfare issue to make sure we manage as well since they potentially witnessed something horrendous."
The detective would not comment on whether police had recovered a knife or what help staff and pupils gave to the injured teacher. But he said the incident happened in a science class.
Speaking outside the school gates, the executive principal of the Dixons Academy group in Bradford, Nick Weller, said: "It happened right at the beginning of the school day and there were some students who witnessed it. A few students witnessed it - in a classroom.
"Those students have been interviewed by the police. Obviously, it's a very shocking thing for them to see. The general atmosphere in the school is very calm, quite orderly, we're trying to keep to normal routines as much as possible."
Mr Weller said there was nothing in school that suggested the attack was going to be launched.
He said: ''I think there was some....the police are looking into the lead up to this incident, there was nothing in school that led up to it - there may have been other people outside the school who maybe knew something.''
He confirmed the suspect is a pupil had been at the school for a year.
Earlier, parent Shakeel Ahmed, 39, said he got a text from his 14-year-old son saying there had been a stabbing.
"My son texted my wife and said 'child stabbed teacher' - that's it," said Mr Ahmed. He then sent a text to say everybody's all right."
Mr Ahmed said: "I came to see my son and see if he's all right but the police wouldn't let me in. The teachers said he's OK."
He said the school was "good" and did not have discipline problems.
But he said: "They should check every child who goes in to check they're not carrying anything. The school should take its responsibilities."
Asked how he felt receiving the text, Mr Ahmed said: "I was scared. I didn't know what was going to happen."
He added: "The school is good but they should have let the children go home."
Another parent, Tahir Jamil, emerged from the school saying he had been reassured that his 15-year-old daughter was safe and meeting staff with around four other families.
''They've explained everything to us now,'' he said. ''Two of the teachers came out and explained everything to us.
''They didn't tell us what teacher it was but now I'm satisfied. I wanted to take my daughter with me but they assured us that the school is safe.''
The was an obvious police presence at the school throughout the day.
Neil Miley, the school's principal, said: "Staff were called very quickly and paramedics were called immediately into the school."
Asked why they kept the school open, he said: "Because I think it's important that we make sure there's consistency for our students, as students need to be safe and secure.
"And there's support from staff on site, making sure our students are looked after, and they've responded with fantastic maturity."
Education Secretary Nicky Morgan said violence of any kind in schools is "totally unacceptable", adding that her department is "ready and prepared to offer all necessary support".
Ian Murch, of the National Union of Teachers (NUT) Bradford branch, which has members at the school, said: "Obviously people are shocked - they would be when an incident like this happens at a school. We hope it can be resolved without any more suffering or difficulties for anybody."
He said the incident "does raise questions with people about safety and what can be done to improve safety".
But Mr Murch also said that attacks remain relatively rare compared with somewhere like the United States in terms of incidents in schools, adding: "We have to hope it stays that way."
:: The school is about 15 miles away from the Corpus Christi Catholic College, in Leeds, where veteran teacher Ann Maguire was stabbed to death by a student just over a year ago.