The bomb exploded as the convoy carrying security personnel, foreign aid workers and journalists was on its way to a school-opening ceremony in Lower Dir district of North West Frontier Province, district police chief Mumtaz Zarin Khan said.
The explosion occurred when the convoy was passing by another state-run girls' school located a few kilometres from Tirmargarha, the main city of Dir district, witnesses said. At least three foreigners and four girl students were among nine people killed in the blast, district administration chief Ghulam Muhammad said.
There was confusion about the identities and professions of the dead foreigners.
The US embassy said it was investigating reports that USAID workers were among the dead.
Initial reports from the remote area had said four foreign journalists were killed in the blast but this was later ruled out by officials.
Forty one girl students, four Pakistani journalists and five security personnel were among the 70 people injured, officials said.
Four of the students were in a serious condition. An emergency was declared in the hospitals in Timergarha.
The roof of the girls' school caved in due to the blast.
Classes were being held outside the building due to the harsh winter weather and this had minimised casualties, district administration chief Muhammad said.
"If the students had been in their classrooms, the number of deaths would have been higher," he said.
The convoy was on its way to the inauguration of a school that was reconstructed after it was destroyed by the Taliban.
Security forces cordoned off the area and launched a search operation to trace those responsible for the attack.
Dir is located between the Swat valley and Bajaur tribal region, where Pakistani troops have conducted campaigns against the Taliban.