"There is no justification for this," said Khan.
Khan said he strongly condemns the attack on the Army Public School on Warsak Road in Khyber Pakhtunkhwa's Peshawar city.
"We are with the families of the victims. All of Pakistan is with them," he said.
The PTI chairman, whose party rules the province, said that the call given by his party for a country-wide shutdown on December 18 has been postponed.
In one of the most gruesome attacks on children anywhere in the world, 8-10 suicide attackers wearing paramilitary Frontier Corps uniforms today entered the school and went from classroom-to-classroom shooting indiscriminately.
The cricketer-turned-politician, who has been holding a sit-in in Islamabad for the last four months alleging rigging in general elections last year, said he was leaving for Peshawar.