Anne Maguire, 61, was taken to hospital by ambulance from the Corpus Christi Catholic College on Neville Road, on the outskirts of the city after midday but was pronounced dead a short time later.
A 15 year-old boy is now being questioned by West Yorkshire Police.
Detective Superintendent Simon Beldon, from West Yorkshire Police, said: "We would like to reassure people that this has been an isolated incident and there is no ongoing risk to pupils or staff at the school.
"The situation is under control and officers, including safer schools officers and members of the local neighbourhood policing team, are currently at the school and are liaising closely with staff.
"The rest of the school is continuing to operate as normal and we would ask that parents do not attend the site unless directly requested to do so by the school.
"Our inquiries are at a very early stage but the full circumstances of this incident will obviously be the subject of a full and thorough investigation."
The school is described on its website as having a "stong Christian and community ethos" and a "very good pastoral structure" where "teachers receive very good back up support".
Over 900 pupils between the ages of 11 and 16 currently attend Corpus Christi Catholic College, which was judged "good with some outstanding features" as part of its last OFSTED inspection in 2010.
A student told reporters outside the school following the incident that the teacher "looked after everyone she could" and always had a "smiling face".
The Corpus Christi Catholic College is on the border of MP Hilary Benn's constituency of Leeds Central, he said: "the school school community, as I would expect is coming together. People whould support one another, and that's what we can do while the police investigate; offer every comfort and help we can to those that who've been affected.
Tom Riordan, Chief Executive of Leeds City Council, said: "We are aware of the serious incident at Corpus Christi Catholic College this morning and our thoughts are with all those concerned.
"We are working with the school to support pupils and staff at this very upsetting time."
Mr Riordan added the authority was working closely with the police and helping with their investigation: "The situation is under control and officers, including safer schools officers and members of the local neighbourhood policing team, are currently at the school and are liaising closely with staff.
"The rest of the school is continuing to operate as normal and we would ask that parents do not attend the site unless directly requested to do so by the school."
The incident comes only days after an investigation by Sky News revealed almost 1,000 pupils were caught with weapons including guns, axes and a meat cleaver in schools in the last three years.
The figures found that 981 children have had weapons confiscated on school premises since 2011.
They included at least 80 primary school children, the youngest of whom was an eight year-old caught with a knife.
Some 36 pupils were found with an assortment of guns, including two hand guns, seven air-powered weapons and 27 BB guns.
Of those found with weapons, 329 caught with items including an axe, a cut-throat razor and a stun gun were charged with a criminal offence.