Lillian Serunkuma and Paul Barnes spoke as their son’s 15-year-old killer was locked up for at least 14 years.
Mr Barnes said: “The reason I wanted it a bit longer was just to deter these kids from picking up a knife and using it on another boy.
“Half the time these boys have arguments and usually it’s something trivial.”
With stiffer sentences, young people would think “it not worth it, let’s have a water fight instead”, he said.
He went on: “As far as I’m concerned if you are dong an adult crime you deserve an adult sentence.
“You hear he’s a kid he’s a kid. You have murdered someone. If you do that, in my eyes, you are not acting like a kid.”
Ms Serunkuma described Quamari as a “lovely boy, very warm, very loving, very funny” who had nothing to do with gangs.
He was hard-working and aimed to get “top marks” in his GCSEs before he was stabbed at outside the gates of Capital City Academy, in Willesden, north London, on January 23.
Ms Serunkuma said there were many reasons for knife crime but the influence of youth culture and rap music were not factors in her son’s murder because it was not gang-related.
She said: “This was a young boy who took it upon himself to take a knife to do damage to someone. That’s his action. It was not a gang-related decision.”
On the defendant’s admission of guilt following his conviction at the Old Bailey, she said: “I appreciate he’s done it but it is a little too late.”
Mr Barnes added: “We had to go through hell. To say that at the last minute, personally, I’m not having it. He could have said that on January 24.”
Quamari Barnes’ uncle, Mark Barnes said after the sentencing: “There are no winners, nobody won today. He got the sentence and he’ll be out before he’s 30. Quamari has no more life.”