The children were aged between 5 and 10 while the slain principal was a member of the Awami National Party, which mainly comprises ethnic Pashtun migrants from northwestern Pakistan, police added.
The assassins fled on motorbikes and nobody has yet claimed responsibility for the attack, police said, adding that investigation was underway.
Violence has escalated in Pakistan ahead of the May 11 election. It is a historic election for it will mark the transition between two democratically elected governments in the country for the first time since 1947. The sectarian attacks against minority Shiite Muslims, including a bombing here which killed 50 people earlier this month, has raised serious concerns over the security situation.