A woman busted for leaving her infant in the car...

Elder being taken away by police.

The privileged San Francisco teenager accused of fatally stabbing a police officer in Rome once pummeled a classmate at a party, causing a severe brain injury, according to a recent report.

Finnegan Lee Elder — who allegedly knifed undercover cop Mario Cerciello Rega to death last week — was 16 when he punched one of his high school football teammates during a late-night bash in Stern Grove park in October 2016, sources told the San Francisco Chronicle.

The victim, who was also 16, struck his head when he fell and was hospitalized with “life-threatening injuries,” police said at the time.

It took a long time for the teen to recover, but he has since graduated high school and is attending college, the article said.

Elder turned himself in after the incident and was arrested on suspicion of battery involving serious bodily injury.

His case was adjudicated in juvenile court, which found that he committed a felony offense, the sources said. It’s unclear what, if any, punishment he faced.

Elder’s uncle, Sean Elder, said the incident was part of a “a mutual pre-agreed upon fight, which many football team members knew about and egged on.”

“I’ve known Finn for his entire life and have never seen him be violent, or even lose his temper,” Elder said, adding that the school, Sacred Heart Cathedral Preparatory School, didn’t discipline his nephew.

Finnegan Lee Elder, now 19, later transferred schools when his dad moved to Mill Valley, his uncle said.

Both Elder and the other teen charged in the cop’s death, Gabriel Christian Natale-Hjorth, 18, graduated from Tamalpais High School in Mill Valley in 2018.

Both boys were on vacation in Italy Friday when they shelled out $100 to a dealer for what they thought was coke — only to learn it was aspirin powder, police said.

The pair stole the dealer’s bag and cellphone in retaliation and demanded the equivalent of $112 plus a gram of coke to return the items, cops said.

The dealer tipped off the cops, who set up a sting near the teens’ four-star Hotel Meridien Visconti.

A brawl eventually broke out between the suspects and undercover cops, including Rega, and Elder allegedly knifed the officer in the chest while Natale-Hjorth punched another policeman.

“Before falling to the ground, [Rega gasped], ‘They stabbed me,’ ” according to cops.

Italian officials said both young men have confessed to the crime. But Elder is now claiming he acted in self-defense when he was grabbed by the neck.