The Tehreek-e-Taliban Pakistan (TTP) has claimed responsibilty for the attack in Peshawar's Army Public School on Tuesday.
TTP spokesman Muhammad Khorasani told AFP there were six attackers. "They include target killers and suicide attackers. They have been ordered to shoot the older students but not the children," he said.
"It's a revenge attack for the army offensive in North Waziristan," he said, referring to an anti-Taliban military offensive that began in June.
"We targeted school because army targets our families. We want them to feel our pain", Reuters quoted the Pakistan Taliban as saying.
Agencies reported that at least 84 students have been killed among over a 100 dead.