Luisa Meco’s 14-year-old daughter, Ivane, has known Harvey since they were in nursery together.
Ms Meco described his death as “shocking”.
She said: “It’s all a bit unreal. I don’t think she’s processed it all properly. It’s a big thing to happen to a local school.
“It’s not something you imagine that your children go to school and something like this happens.
“It’s really shocking, it”s really sad.”
Other mourners gathered outside Sheffield United’s stadium Bramall Lane. Harvey was reportedly a fan of the football club.
A club spokesperson told Sky News: “All at Sheffield United were saddened to learn of the passing of Harvey, our deepest condolences are offered to his family and friends at this difficult time.
“We share the shock and distress of the community following this tragic incident and appreciate that discussions are underway with regards to tributes to remember Harvey.”
Speaking on Monday afternoon, Assistant Chief Constable Lindsey Butterfield told a news conference: “We know that what has happened will cause significant distress and concern.
“I would like to reassure you that our officers will remain on scene and in the local area to offer reassurance to parents, staff and local residents as our investigation continues.”
Image: Pic: PA
Sir Keir Starmer, who was in Brussels at a dinner with EU leaders on Monday, said: “It is horrific and senseless news that a boy has died after a stabbing in Sheffield.
“Our schools should be places of safety and learning, not violence and fear. My government is absolutely committed to tackling knife crime so young people are safe on our streets and in our schools.
“Tonight a young boy should be returning to the love and safety of his family.”