At 8.55am today, a 50-year-old male teacher was rushed to hospital after being stabbed in the stomach at the school in Bradford.
He is currently said to be in a stable condition.
Police are now searching for the 14-year-old student, who is described as Asian, with short dark hair, after he fled the scene.
He was last seen wearing a blue school blazer.
Detective Superintendent Simon Atkinson, of the Homicide and Major Enquiry Team, said:
"There were a number of pupils who witnessed this incident, which took place in a classroom, and they are currently liaising with 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," he said.
The incident has brought back horrific memories for many, as it took place less than 20 miles from the school where 61-year-old Ann Maguire was stabbed to death by pupil Will Cornick last April.
Ian Murch, of the Bradford NUT (National Union of Teachers), told BBC Radio Leeds: "It does raise issues about knives in school.
"You rarely get incidents of this kind but we do have to understand there have been two incidents of this kind now."
"This is very difficult indeed."