The bus was ambushed on the outskirts of the northwestern city of Peshawar yesterday, also killing the driver in a hail of bullets and rocketfire.
Tehreek-e-Taliban Pakistan (TTP) claimed responsibility for the attack saying it was the work of their Khyber chapter, AFP reports.
The boys, aged nine to 14, from Khyber School, were targeted in the Matani area close to Pakistan's tribal belt, which the United States considers the most dangerous region on earth, and which has an al-Qaeda headquarters, The Australian reports
Bombings blamed on Taliban and al-Qaeda-linked networks have killed more than 4630 people since 2007.
"Gunmen opened fire on a school van and also lobbed a rocket in Peshawar's suburb of Matani," senior police official Ejaz Khan told AFP. "We are checking why the bus was targeted."
Habib Khattak, a doctor at Peshawar's Lady Reading Hospital, said 18 wounded were admitted after the attack, 12 of them children as well as teachers and passers-by.
Police said the bus was taking children home at the end of the school day, in the early afternoon.
"The gunmen were waiting for the bus in fields and attacked when it came close. They fired a rocket and then fired bullets on the van," said a senior police official, Kalam Khan.
In the district of Lower Dir, bombers yesterday assassinated Sher Khan, a local leader of Pakistan's main ruling Awami National Party in the northwest, blowing up his vehicle.
Lower Dir borders the once Taliban-infested Swat valley, where a sweeping army offensive two years ago appeared to reverse the local Taliban insurgency.