Police said all teachers and children at the high school for boys were safe after the gunmen killed the guard in the central town of Dera Ghazi Khan, 500 km southwest of Islamabad.
"We are still not sure how many gunmen there were and whether they are still there. The police are searching," district police officer Rehmat Ullah Khan Niazi told Reuters.
Read: Assailants hurl hand grenade at school in Karachi
Tehreek-e-Taliban Pakistan militants killed 153 people, most of them children, at a high school in the northwestern city of Peshawar on December 16.
There have been two grenade attacks on schools in Karachi since early February though no one was hurt in either one.
Read: Karachi school comes under grenade attack
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