Emergency services were called to Wellesley Road in Croydon at around 8.30am to reports that a 15-year-old girl had been stabbed.
Both the land and air ambulance attended but she was pronounced dead at the scene 50 minutes later.
It is believed that the teenager arrested knew the victim and he currently remains in police custody.
Speaking to journalists at the scene, Metropolitan Police Chief Superintendent Andy Brittain said officers arrived within two minutes of being called.
He said: “This is every parent’s worst nightmare, and I know the officers who responded this morning, along with our emergency service colleagues, are devastated at the victim’s death.
“This is an emotion I share and I know people across Croydon will be feeling the same.
“The victim’s family has been informed and our thoughts are with them at what must be an incredibly difficult time.
“We carried out urgent inquiries to find the suspect and within 75 minutes of the incident happening a 17-year-old boy was arrested in New Addington.
“He remains in custody and will be questioned by detectives.
“We remain in the early stages of our investigation, however based on what we know so far we believe that we are not looking for anyone else in connection with this offence.
“From our initial inquiries, we believe the suspect may have known the victim. However, we’re not in a position to release the victim’s identity at this time.”
A woman has described seeing a bus driver and another woman trying to save the 15-year-old girl, who was a pupil at Old Palace of John Whitgift School.
The school released a statement to say: “We are deeply shocked by the senseless and tragic death of our much-loved and valued friend and pupil.
“It will take some time for the Old Palace community to come to terms with this terrible news, and we will offer support to our pupils as we try to do so.
“Above all, we send our love and deepest sympathies to the girl’s family at this unimaginably distressing time.”
James Watkins, from Mainz World which runs prevention and intervention projects for children and young people, was supporting the family of the 15-year-old girl who was stabbed to death in Croydon this morning and said they are “devastated”.
He said he was with the girl’s family at the scene between 9am and 10am.
He added: “I came down to support the family and some of the young people.
“We were with the family this morning. They were devastated and I would say more importantly in shock, I don’t think it feels real for them.”
Mr Watkins said the girl’s family was contacted by a friend of the girl.
Witnesses or anyone with information can contact police on 101 quoting reference CAD 1601/27Sep, to remain anonymous contact Crimestoppers on 0800 555111 or online.
https://www.thisislocallondon.co.uk/news/23817878.boy-arrested-new-addington-girl-dies-croydon-stabbing/?ref=rss Boy arrested in New Addington after girl dies in Croydon stabbing