David Cameron has paid tribute during Prime Minister's Questions to Ann Maguire, the teacher stabbed to death in a classroom, calling it a "truly shocking and appalling tragedy".
The Prime Minister told MPs: "It is clear from the tributes paid she was a much-loved teacher who'd worked at the school for over 40 years.
"She cared so much about her pupils she'd come in on her day off to help prepare them for exams."
Police have been given extra time to question a 15-year-old boy over the incident.
Police have been given extra time to question a 15-year-old boy after a teacher was stabbed to death in front of her pupils.
Spanish teacher Ann Maguire, 61, was attacked in a classroom at Corpus Christi Catholic College in Leeds on Monday.
West Yorkshire Police have confirmed the teenager, who was detained by staff following the stabbing and arrested at the school, remains in custody.
Officers said they were given permission to continue to hold the boy by magistrates in Leeds last night.
Flowers and messages are still being left outside the school where a Spanish teacher was stabbed to death during a class.
Ann Maguire had taught at Corpus Cristi Catholic College for 40 years. She died on Monday.
A community remembrance service will be held this morning.
A 15-year-old is still being questioned by police.
Tributes have been paid to the teacher stabbed to death at a school in Leeds yesterday.
Ann Maguire was "an inspiration" to both pupils and staff, and was due to retire from Corpus Christi Catholic College in September.
The school re-opened today, and a fifteen-year-old boy remains in custody, as ITV News' Martin Geissler reports:
The Archbishop of Westminster has spoken of his "utter shock and sadness" over the fatal stabbing of teacher Ann Maguire.
Vincent Nichols, the leader of Catholics in England and Wales, who was made a Cardinal earlier this year by Pope Francis, said his heart went out to the family of Mrs Maguire.
He said: "Today I express my utter shock and sadness at the tragic killing of Mrs Ann Maguire at Corpus Christi College.
"My heart goes out to her husband and daughters, to their wider family, to everyone at Corpus Christi Catholic College and to the parishes connected to the College."
Current and former pupils have praised a much-loved teacher who was stabbed to death in front of her students.
Paying tribute to Ann Maguire, one teenager told ITV News, "She helped me and I'm on the right pathway, I'm studying acting at Park Lane College now and I'll be going to uni soon."
"Without her I don't know where I'd be," he added.
Ann Maguire, the teacher stabbed to death at a Leeds school, would have "strongly opposed" making pupils pass through metal detectors as they arrive, her headteacher has said.
Steve Mort, head of Corpus Christi Catholic College, said he did not think detectors were necessary as the school was a "safe environment".
"I would want to remind people that this is an isolated incident. It's a tragedy. It's unprecedented. It wouldn't be in keeping with the community", Mr Mort said.
"I think ultimately Ann Maguire herself, if she was asked that question, would strongly oppose any suggestion that we should have metal detectors in this school."
The headteacher at Corpus Christi Catholic College said the teacher stabbed to death at the school was "an inspiration" to both pupils and staff.
Steve Mort said: "Ann Maguire was a wonderful, caring individual. She was an outstanding teacher and she was an outstanding leader.
"She had been a cornerstone of the school community for 40 years, she touched the hearts of three generations of pupils.
"I know particularly at times like we're experiencing at the moment she would have been at the forefront of supporting us."
A former pupil of Ann Maguire said today she had lost a friend.
Mrs Maguire taught for forty years at Corpus Christi Catholic High School. She died yesterday after being stabbed.
Kathleen Walpole, her brothers and three children were all taught by the 61 year old:
West Yorkshire Police said the fatal stabbing of a teacher was "very much an isolated incident".
Chief Inspector Nik Adams, responsible for neighbourhood policing in the area, said, "This is not part of a wider problem that exists locally."
"This school has absolutely no issues in terms of weapons", he added.