A 15-year-old boy has died after being stabbed at a school in Sheffield, police have said.
The boy, named locally as Harvey Willgoose, was injured at All Saints Catholic High School, in Granville Road in the city, at about 12:17 GMT.
Police and paramedics were called but the teenager died a short time later.
A 15-year-old boy has been arrested on suspicion of murder and remains in police custody, South Yorkshire Police said.
Assistant Chief Constable Lindsey Butterfield told a press conference the force was working "at pace to build a full picture of what had happened and how this tragedy unfolded".
"We urge you to be mindful that there are loved ones at the centre of this," she added.
A large police presence remained at the school and in the immediate area.
All Saints head teacher Sean Pender had sent a message to parents on 29 January about a lockdown at the site.
In it, he said the action was due to "threatening behaviour between a small number of students".
ACC Butterfield was asked during the news conference about the reports but the officer said she could not share any information relating to it.
She concluded the press conference by urging people to avoid speculation and the sharing of content online "which could be distressing to them [the family] and detrimental to our investigation".
A number of floral tributes and messages have been left outside the school this evening.
One of the messages described the teenager as someone who was "the life of a party" who "brought joy and laughter to everyone who knew him".
It adds that his "bubbly personality was contagious".
"I'll never forget all the moments, all the laughs. You'll be missed by so many," it said.
A teenager who said he knew the 15-year-old also left a bunch of flowers outside the school.
The 17-year-old, who studies at a local college, heard the news through friends and said the boy who died was a fan of Sheffield United.
He said: "I'm a bit upset to be honest, it came as such a shock.
"He seemed quite a happy guy and a nice guy to be around."
One All Saints pupil said the victim had been a "lovely lad".
"He would make your day better hearing him down that hallway. You didn't have to personally know him to be upset," the 17-year-old said.
"Teachers loved him, students loved him, everyone loved him.
"I've been at the school for seven years now and nothing like this has ever, ever happened."
Asked about the incident while on a trip to Brussels, Prime Minister Sir Keir Starmer said his first thoughts "as a father" were with the victim's family.
"The terrible stabbing in Sheffield is one where I think the whole country would want to reach out to the family, to the friends, to the school, the entire community in their grief and mourn with them," he said.
Louise Haigh, the MP for Sheffield Heeley, said the news was "horrific" and her thoughts were with all affected.
"Serious questions will have to be answered about how this could have happened", she said, adding she will work with the school, police and the council "to make sure they are".
Education Secretary Bridget Phillipson said she was "devastated" by the boy's death.
She added: "We are in contact with the school and council to offer support."
In an emotional statement education minister Catherine McKinnell told the Commons: "My heart goes out to his family, friends and the entire school community at this very distressing time."
South Yorkshire Mayor Oliver Coppard said he had spoken with Lauren Poultney, the county's chief constable, and offered her his full support.
"This morning a teenage boy went to school like thousands of others across South Yorkshire but won't come home; a young man who was a member of our community, with his whole life ahead of him," he said.