"This case is extremely emotional," lawyer Lonny Fish, who represents 17-year-old Kinta Stanton, told jurors. "The Conroy family was turned upside down by childish antics."
The death was the first of several unprovoked subway assaults that alarmed the city last year.
Stanton - along with co-defendants Ameer Best, 18, and Nashir Fisher, 17 - are on trial just blocks from the site of the March 2008 attack near City Hall. Two other teens have pleaded guilty to third-degree murder and may testify.
All five had skipped school in North Philadelphia that afternoon for a jaunt to a downtown mall, Assistant District Attorney Jacqueline Coelho said.
They were on the subway platform when someone challenged another group member to hit someone, she said. Arthur Alston took the bait, randomly punching Conroy from behind.
A police officer nearby soon observed four teens beating and kicking the man. By the time the officer got to him, Conroy was on his knees, gasping for breath. He was pronounced dead within the hour.
To Coelho, the case comes down to a simple childhood adage: keep your hands to yourself.
"When you put your hand on another human being, you are responsible for what happens to that person," she said in opening statements.
Defense lawyers challenge the conspiracy and murder charges on several grounds.
Stanton and Fisher maintain they never touched the victim, and were never part of any conspiracy. Best's lawyer, Richard Brown, suggested he will revisit the autopsy report to explore whether allergies also contributed to the death.
"The physical injuries alone would not have killed Mr. Conroy," Brown said.
Conroy had talked by phone with his fiancee before setting out to check on the store, adjacent to a downtown Marriott hotel. Employees there described Conroy as kindhearted and a bit of a jokester, albeit one who took his job seriously.
"If he saw you were a little down, he would joke around with you," Jasmine Miller, 19, said Monday as she worked the counter. "He didn't want anyone in his presence to be unhappy."
A court-issued gag order prevented Conroy's family or the defendants' relatives from speaking to the press.
The trial is expected to last about a week. The third-degree murder charge carries a maximum 20- to 40-year sentence.
Alston, 16, has pleaded guilty to third-degree murder, admitting he threw the first punch. He is set to be sentenced next month. His bail was revoked after he allegedly punched someone at an alternative school in April.
Stanton has also been arrested again, for allegedly shooting a friend while on house arrest. A judge then revoked a decision to try him in juvenile court in Conway's death.
Rasheem Bell, 17, is also awaiting sentencing next month after pleading guilty to third-degree murder. His lawyer has called him very remorseful for his role in Conway's death.
