Members of the public came to help him and called the emergency services but Leo died in hospital.
His family said he had been walking a 10-minute route home when he was attacked and described him in a tribute as “funny, sweet and had not one aggressive bone in his body”.
The defendant, who was 14 at the time of the stabbing, also pleaded guilty to two counts of causing grievous bodily harm with intent and one of assault occasioning actual bodily harm, over separate attacks on three elderly women.
Those victims were pushed to the ground, hit by him and suffered serious injuries, the Crown Prosecution Service said.
