Government spokesman Mushtaq Ghani told CNN that the gesture was a way to remember the bravery of the students who were taking exams in an auditorium when Taliban militants strapped with explosives laid siege.
Some six hours later, 145 were dead, 132 of them children, according to authorities.
In February, Pakistani authorities arrested a suspect, accusing him of commanding the attack.
The Army Public School is home to about 1,100 students and staff, most of them sons and daughters of army personnel from around Peshawar.