The teen arsonist accused of starting a fire that killed a cop used an aerosol can and lighter to blast fireballs under a cafeteria table at his middle school when he was just 13 years old, law-enforcement sources told the Post after the teen’s pretrial hearing Tuesday.

Marcel Dockery, now 17, was punished with court-mandated fire safety classes and even sat on his couch with his parents for 90 minutes while a FDNY fire marshal lectured him about fire safety and prevention, the sources said.

A Dockery pal who was also arrested for using an orange lighter and a can of “Hatworld Lids Perfect Curve” to blast fireballs at their Coney Island middle school in 2010 also told prosecutors that Dockery regularly tried to spark blazes.

“He told prosecutors that Dockery is always trying to light things on fire. Whether it’s a doorknob or a banister or anything, he’s always trying to light a fire while they’re hanging out,” a law-enforcement source said.

Dockery was in court Tuesday for a pretrial hearing on the murder rap he faces for allegedly setting the Coney Island fire that killed NYPD police officer Dennis Guerra last year.

The teen – who said he started the fire because he was “bored” and flashed a huge smile to reporters after he was arrested – didn’t speak in court.

Guerra’s family said outside court that the teen should rot behind bars forever.

“I want justice to be served for what he did. He took my son’s life and almost took another life,” said mom Miriam Guerra, 65.

“I want him to be put behind bars for the rest of his life,” said widow Cathy Guerra, 38.

NYPD Det. John McDonald revealed on the stand that Dockery took court-mandated fire safety classes after an arson incident at school.

Under questioning from assistant district attorney Howard Jackson, the detective also testified about how Dockery initially lied and said the fire was already burning when he took the elevator up to the floor of his aunt’s apartment

But Dockery confessed when confronted with contradictory evidence like the surveillance footage from the elevator that showed no smoke and Dockery calmly walking off onto the floor when the doors opened, the detective said on the stand.

Four uniformed cops from Guerra’s command sat in the Brooklyn Supreme Court gallery Tuesday to show support and comfort the family.

Police officer Rosa Rodriguez was also severely injured in the fire and hospitalized for weeks but lived.

Dockery defense attorney Jesse Young did not return a call for comment.