Charges against the men were dropped in 2018 following the fatal stabbing in Betts Park the year before.
But detectives say they continued working to ensure justice for 17-year-old Michael Jonas and his family.
The six found guilty include 23 year old Divon Henry-Campbell from Streatham and 21 year old Ryan Girense Brown from Croydon.
Detective Chief Inspector Matt Webb, from the Met’s Specialist Crime Command, said: “Michael was just 17 when he was fatally stabbed in a violent, pre-planned attack. His family remain devastated by his loss and will never come to terms with what happened that night.
“This has been a long and complex investigation but we were determined to keep fighting for justice and we are pleased these men will now be off the streets of London and unable to cause further harm.”