Abedi was part of an Arabic-speaking “clique” during his time at the school, The Times reports.
He is believed to have been part of a group of teens that became upset when one of their teachers brought up the topic of suicide attacks.
The teacher “asked what they thought of someone who would strap on a bomb and blow people up”, according to a source quoted by the paper.
The source said the boys then went to their RE teacher and lodged a complaint, telling them it was “Islamophobic”.
The source added: “[Abedi] was a silly boy, not very serious. He was not smart enough to be a mastermind of anything like that.”
A spokesperson for Burnage Academy said yesterday: “We feel the pain that Manchester feels. We stand shoulder to shoulder with our fellow Mancunians against terrorism in all forms.”
Pals of the evil killer yesterday revealed his wild youth of booze and drug-taking – and how he was nicknamed “Dumbo” because of his big ears.
Pictures show him grinning in a bar with three student friends and on a beach in Libya, before he is thought to have been indoctrinated.
Pals described the brainy Manchester United fan as “very jolly”.
But over the past two years he is said to have changed completely after a number of trips to Libya to visit his family.
His parents are even said to have confiscated his passport amid fears he was being radicalised.
This week it was claimed Abedi called his family in Tripoli 15 minutes before the attack.
His mother Samia Tabbal, 50, and father Ramadan, 51, a security officer, were born in the Libyan capital.
They emigrated to London before moving to Manchester.
Manchester-born Abedi is believed to have regularly travelled to see his family, who moved back to Tripoli following the fall of Colonel Muammar Gaddafi in 2011.
One friend said: “He became silent and withdrawn after a trip a couple of years ago. Before then he was a happy-go-lucky kid and always did well in school.
“We used to party together. He loved being around his friends and wasn’t a strict Muslim at all.
“He even used to drink alcohol and loved smoking weed, he never mentioned religion.