One of five North Philadelphia teenagers whose unprovoked attack inside a Center City subway concourse last year resulted in the death of Sean Patrick Conroy was sentenced yesterday to 121/2 to 25 years in prison.

Arthur Alston, who had pleaded guilty to third-degree murder, seemed stunned when Common Pleas Court Judge Jeffrey P. Minehart imposed the same sentence on him that he had on three accomplices convicted at trial.

"I'm not a murderer. I'm not a monster," the 18-year-old told the judge. "I'm a kid who made a mistake."

Actually, Minehart said, Alston made two mistakes. His second was assaulting a student April 21 while under house arrest awaiting sentencing in the Conroy killing.

Assistant District Attorney Jacqueline Juliano Coelho said Alston was at Delaware Valley High School, an alternative school in Philadelphia for students with behavioral problems, when he choked another student until that student lost consciousness. Alston pleaded guilty to that assault yesterday.

Alston and Rasheem Bell, 18, pleaded guilty before this summer's trial of their three friends in the March 26, 2008, attack in which Conroy, 36, was jumped and beaten.

Bell, who unlike Alston testified at trial for prosecutors, is to be sentenced Jan. 4.

Minehart sentenced the convicted - Nashir Fisher, 17, Emir Best, 18, and Kinta Stanton, 17 - on Oct. 29.

According to trial testimony, the five Simon Gratz High School students cut class about lunch time March 26, 2008, and went to the Gallery at Market East in Center City.

Later, they met a friend who dared them to hit the first person they saw in the subway. Conroy, the manager at a Center City Starbucks, was that person. Alston began the attack, rushing up behind Conroy and punching his head. The others followed.

By the time police got to Conroy, his lungs were so overinflated from an acute asthma attack that he could not speak. He died at a nearby emergency room.

Yesterday, defense attorney, Ronald J. Pressley, argued that by pleading guilty Alston had taken the first step toward rehabilitation. "He's a bright guy, but he feels a need to prove himself in the most destructive way," Pressley said.

Conroy's mother, Sharon, said in a victim-impact statement that Alston had taunted her and her family from the first hearing, laughing in court and on one occasion approaching her and asking: "How are you doing today?"

Unlike the three sentenced Oct. 29, Alston did not apologize to Conroy's family.