Brian Ludwig's 15-year-old son was buttoning his pants after gym class in the Roxborough High School locker room Monday morning when they grabbed him.
"They picked him up, ripped off his pants, ripped off his underwear. The more he fought, the worse it got," Ludwig said.
The three attackers - all juniors - dragged the freshman around the floor of the locker room and snapped pictures of him on a cell phone, Ludwig said.
School District officials said they are investigating the incident. Police are also investigating.
The three students were suspended for five days, and the school is moving to have them transferred to a disciplinary school, said a source at Roxborough High who has knowledge of the incident.
"We were appalled, and we're dealing with it," the source said.
The district issued a brief statement and sent a letter home to parents. Principal Stephen Brandt did not return calls for comment.
"As soon as school officials were informed of the incident, the school took immediate action to provide support to the affected student and started an investigation," the statement to parents said.
Ludwig remains outraged that his son endured such humiliating treatment at the hands of classmates.
Ludwig said his son at first "thought it was a game, but once his button popped and his underwear came off," he feared he would be sexually attacked.
Ludwig's mother-in-law took the teen to the Police Department's Special Victims Unit to report the attack, he said.
"We're going all the way with this," Ludwig said. "We want to prosecute to the fullest extent.
"These kids are like terrorists. My son's terrified. I'm going through hell now trying to get him to go back to school."
The Special Victims Unit is investigating the case, Officer Michael O'Brien said. The students could face charges of harassment, reckless endangerment, and simple assault, he said.
If cell-phone pictures were put on the Internet, the three could face additional charges, he said. But there is no evidence of that at this point, he said.
O'Brien said the victim reported that the attackers were trying to give him a wedgie and it got out of hand.
Ludwig said his other son, also a junior, was sent home from Roxborough on Monday after the attack.
Neither boy returned to school Tuesday, he said, adding that he's not sure he will ever send them.
O'Brien said he is investigating whether two other students also were targeted.