Police said Corpus Christi had a dedicated 'safer schools' officer stationed in the school, tasked with keeping pupils and staff safe, but she was elsewhere when the attack took place.
At a press conference outside the school, DS Simon Beldon, of West Yorkshire police, said questioning the suspect would take time.
He said: "Given his young age, this is a process which needs to be handled very sensitively, and may take some time to complete."
It is thought to be the first time a teacher has been stabbed to death in a British classroom, and the first killing of a teacher in a school since the 1996 Dunblane massacre.
Recalling his last encounter with Mrs Maguire, Mr Mort said: "I last spoke to Ann just after the staff briefing yesterday morning and found Ann to be her usual self – she was bubbly, she was a character who enjoyed being in the school community ... She was enjoying the latter end of her career although I know, to Ann, that her service to Corpus Christi was not a career. She considered it to be a vocation."
The head said about 750 of the 1,000-strong student body turned up for lessons, which was 'a real sign of solidarity among the students and the staff of the school'.
