The bomb fixed in the cooker was planted outside Foundation Public School in Bashirabad area of Peshawar city of Khyber Pakhtunkhwa.
The bomb disposal squad defused the bomb, a police official said.
Some unknown armed men gave the pressure cooker to a boy to keep it inside the school. The boy, however, kept it in front of the school, he said.
The police said that it was not known yet that either school was the target or the owner of the school.
On January 20, heavily-armed militants stormed the Bacha Khan University and opened fire on students and teachers, killing 21 people.