Leighton, 18, died and several other classmates at Mother Teresa Catholic High School were injured on May 26, 2011, when the sparks created by the cutting ignited cleaning fluid in the barrel.
The explosion blew the lid of the barrel through the ceiling of the shop class and gave Leighton severe head trauma. He died later in hospital.
Former shop teacher Scott Day, speaking publicly for the first time about the incident, said while the class was an automotive shop class, he had suggested to students the idea of making barbecues from barrels.
Leighton and classmate Adam Soliman had expressed interest and successfully made a barbecue from a barrel that had been used to keep scrap metal in the shop. It did not have a lid.
The barrel Leighton was working with on May 26 was one of 11 Day had purchased on Kijiji.
Day said on May 26 the plan was to measure and mark the barrel and come up with a plan to proceed.
'There was no talk of cutting the barrel'
"There was no talk of cutting the barrel," said Day. "On that day, there was no plan for cutting whatsoever."
Leighton was marking the barrel when Day said he walked outside the shop to meet with other students working on a car.
He said he was in a car when the explosion happened.
"My initial reaction was, 'What the hell was that?'" said Day. He said he thought the students in the shop were "messing around."
He found Leighton lying on the floor next to the barrel with the hand grinder still running.
"I didn't realize the severity of what happened," said Day. "I said, 'Get up off the floor.'"
When he realized Leighton was hurt, he said he instructed Soliman to call 911.
"I am very sorry it happened," Day told the inquest.
Classmate didn't hear Leighton get teacher approval
Earlier Tuesday, Soliman told the inquest he had cleaned the barrel with "engine cleaner" days before and had left it to dry. At some point, someone had put the lid back on the barrel.
Soliman said when Leighton started cutting into the barrel, it created sparks. Soliman said he heard "a buckling noise" and turned around, then the explosion happened.
"My ears were ringing, it was very dusty," he said, adding he had to go to the Children's Hospital of Eastern Ontario after he lost hearing in one ear for a few hours.
Soliman said he didn't receive permission to cut and he didn't hear Leighton ask for or receive permission before he cut.
"We weren't allowed to do anything unless he (Mr. Day) said so," said Soliman.
Barbecue project not part of curriculum
Day said he did not have experience making barbecues from closed containers but had seen the project completed at other schools.
He said he knew enough to know the caps should have been removed from the barrel before proceeding.
He also said he was not aware of an approval process in place for special projects such as the barbecue project.
On Monday, Ottawa Fire Service investigator William Hay said cutting into an enclosed barrel is against the fire code and said if any form of solvent or agent is inside a vessel like a barrel, tests should be done to see if there is any of the volatility left inside.
Leighton's family have filed a $400,000 lawsuit against Day and the Ottawa Catholic School Board.
They allege both the teacher and board should have seen the danger of doing "hot work" on a barrel that once held combustible gas.