The stabbing happened around noon at Castor Gardens Middle School in the 1800 block of Cottman Avenue.
Police say the student pulled a knife out of his bag while inside a classroom.
"The other students notified the teachers. Two of the aides went over and disarmed the student. While they did that they were cut superficially," said Deputy Commissioner Frank Vanore.
The victims are a 31-year-old woman and a 63-year-old woman. Police say they suffered cuts to the arm and stomach.
The principal described the injuries as "minor."
No students were injured in this incident.
Police and Philadelphia School District officials say the knife did pass through a metal detector and was not detected.
"The Office of School Safety is currently investigating this metal detector. They are inspected on a routine basis and checked for malfunctions," said school district spokesperson Monique Braxton.
The 11-year-old has developmental issues, police say, and officers are working with his parents.