A group of journalists and aid workers was travelling in an army convoy to the opening of a girls' school in the Lower Dir area when it was hit.
At least 45 people, including school girls, were injured in the blast.
Pakistani troops carried out a major offensive in the area last year to crush Taliban militants.
The BBC's Mark Dummett says the convoy was heading to the inauguration ceremony of a new girls' school, but that the blast occurred near a different school.
A powerful device was detonated as the convoy drove past a school in Koto, a heavily populated village along the route, injuring several children there.
The convoy was on its way to Maidan, an area of Lower Dir district in North West Frontier Province, which is the base of a pro-Taliban cleric Maulana Sufi Mohammad, and a stronghold of Taliban militants.