An assembly on combating racism and urban violence at Masterman High School was supposed to feature a skit with video about someone shot over a pair of sneakers.

Yesterday after performing the skit, the students at the city's top academic magnet high school described how their skit came up against reality on Monday when 11 members of the African American Cultural Committee were attacked while videotaping outside the building.

Raffi Greenberg, 18, who was in the audience, said students were stunned to learn that the attack was over a pair of sneakers. "This actually happened because a pair of sneakers was stolen.

"These people came looking for the person who stole the sneakers," the senior from Mount Airy said. "It's a strange coincidence. It's really ironic."

Three Masterman students were injured, including an 18-year-old senior who was treated for head wounds and released from a hospital. The student, who was not identified by the school or police, needed stitches for injuries he suffered when he was struck from behind and hit his head. He described the unexpected assault during yesterday's assembly.

Bryan Stephenson, a junior from West Philadelphia who knows the students who were jumped, said the attack made the program more powerful.

"It affected us a lot more," Stephenson, 17, said. "It had more of an impact."

Classmate Jordan Gifford, 16, of South Philadelphia, agreed. "I think it came closer to home because . . . the kids who went through it were the kids who were doing the performance."

"Everyone seemed so sincere and heartfelt," said Sam Margulies, a senior from East Falls. "You could tell it was not rehearsed."

Police arrested an 18-year-old Benjamin Franklin High School student and two 13-year-old Waring School students for their alleged roles in the attack, which occurred shortly after 4:30 p.m. Monday at 17th and Spring Garden Streets. At that time, the Masterman students were shooting video for their skit.

A police spokeswoman yesterday could not confirm the Masterman students' account that stolen sneakers were behind the attack.

District spokesman Fernando Gallard said it appeared the assaults were the result of a case of mistaken identity.

The youths pointed at the Masterman students and asked one another, "Is that him? Is that him?" Gallard said, and then they began punching three of the students.

"They seemed to be looking for someone," Gallard said, "and when they didn't find him, picked a fight anyway."

Officials declined to identify the students who were beaten or to release the names of other members of the extra-curricular club who witnessed the attack.

Masterman's academic program spans fifth through 12th grades and draws its 1,200 students from throughout the city. The high school program was ranked 53d in the nation by U.S. News and World Report in its December story on the nation's top 100 public high schools.

Additional police officers and members of the Philadelphia Anti-Drug, Anti-Violence Network were posted outside the building at arrival and dismissal yesterday.

"I never thought I would see this here," said Gwen Enoch of Northeast Philadelphia, who was parked outside the school as usual, waiting to pick up her 11-year-old granddaughter.

"This concerns me," said Rick Meketon of East Mount Airy, who was waiting for his daughter Miranda, 12, and a few other students who are part of their carpool from Northwest Philadelphia. "Most of these kids are not necessarily street-wise. This is an urban school, but these are not necessarily urban kids."

The school at 17th and Spring Garden Street is across the street from the Community College of Philadelphia and only a few blocks from the Spring Garden subway stop at Broad Street.

Assistant principal Mary Ann Tancredi said students from several other nearby schools congregate at Broad and Spring Garden to catch a bus or the subway.

"It is a kind of hub there," she said.

Tancredi said many parents had called the school yesterday to express concern about the attacks.

She said students were sent home with a letter from Principal Marjorie G. Neff explaining the facts and recommending steps to keep students safe.

Parents were urged to make sure that students not involved in after-school activities were picked up at dismissal and that students staying after school were picked up at designated times.

In the letter, Neff said, "Families who are picking up students from play practice or athletics should reach out to families relying on public transportation."