Police said a post-mortem has shown teacher Ann Maguire died of multiple stab wounds.
The 15-year-old suspect remains in custody and will be interviewed at some time today, West Yorkshire Police said.
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.