Police have said Anne Maguire, who was fatally stabbed at her college in Leeds, suffered "a number of stab wounds" in the attack.
A knife has been recovered from the scene by police, and a 15-year-old pupil is being held on suspicion of the stabbing.
The alarm was raised by students in the school, some of whom witnessed the attack, said a police spokesman.
The 15-year-old pupil was detained at the scene by other members of the teaching staff immediately after the attack occurred.
The police spokesman said the stabbing was "an unprecedented event" in Leeds, "the likes of which I've not seen in over 25 years of police service".
Although a shocking attack, this is a rare attack. There are few other occasions when a teacher was stabbed inside their school by a pupil.
A 15-year-old schoolboy is in police custody tonight, suspected of stabbing to death his teacher.