Several militants wearing black dress entered Army Public School on Warsak Road by scaling its walls and started indiscriminate firing, a senior official said.
A police official said that the school has been cordoned off, with a rescue operation in progress. About 500 students and teachers were believed to be inside at the time of attack.
A spokesman for a Tehreek-e-Taliban Pakistan faction has claimed responsibility for the attack.
An official said that about six militants were involved in the attack. The attackers were wearing black uniforms.
Still hundreds of students are still trapped inside.
Several students escaped through the back gate. One of students who escaped told Dunya TV that fourth period was in progress when they heard firing.
“First we didn’t know what has happened. But late an army officer told us to escape through back gate,” he said.
The TTP spokesman Muhammad Khorasani said that the six attackers were asked to target the older children but not the younger ones.
Pakistani Prime Minister Nawaz Sharif condemned the attack saying that the terrorists should not be spared and that the kids were the foremost priority at the moment.
Police said that the terrorists stormed inside the school after scaling its walls.