The Friday’s attack targeted police guarding a polio vaccination drive in Mastung, a town in Balochistan province.
“The target was a police van which was going to pick up a polio (vaccination) team,” Senior Superintendent of Police Rahmat Ullah told the Reuters news agency.
One police officer and a shopkeeper were also killed in the explosion, senior police officer Abdul Fatah told the AFP news agency.
The blast was believed to have been caused by an improvised device attached to a motorcycle parked near the school, Al Jazeera reported.
No group has claimed responsibility for the attack. But Islamabad has frequently blamed the Taliban government in neighboring Afghanistan for failing to root out attacks on Pakistan from across the border.