The boy was interviewed and taken to Children's Hospital of Philadelphia to be examined. The Special Victims Unit determined the case to be founded.
The attacker was described as a 13- or 14-year-old black male who was wearing a red shirt and blue pants that appeared to be a school uniform, police said.
Fernando Gallard, a spokesman for the Philadelphia School District, said the 12-year-old boy has attended McMichael, 3543 Fairmount Ave., for several years.
There is no safe-corridor program in effect at McMichael, which is not one of the schools taking new students as the result of school closures.
In August, Mayor Nutter and Superintendent William R. Hite Jr. announced an expanded program to ensure that students are safe on their trips to and from city schools.
The WalkSafePHL initiative involves district, city, and SEPTA transit police, and community members to protect students as they walk, bike, or ride public transportation between home and school.
The focus of the initiative is on roughly 10,000 students who were transferred because their old schools were closed.
Monday night, Hite called the assault "a horrendous incident."
Speaking after a planning meeting of the School Reform Commission that was devoted to school safety, Hite said: "We have had those issues before - unfortunately. We all have to be vigilant about our children. We're going to work very closely with school security, school police, and everyone else to ensure our children are safe on the way to and from school."
Tuesday afternoon, Hite and Nutter are scheduled to walk with children from Ethel Allen Elementary School as part of WalkSafePHL.
"That was already planned," Hite said. "This incident makes tomorrow's walk even more important."
Contact Robert Moran at 215-854-5983 or firstname.lastname@example.org, or follow @RobertMoran215 on Twitter.
Inquirer staff writers Martha Woodall and Sulaiman Abdur-Rahman contributed to this article.