The blast shook a madrassa in Quetta, where the students had just concluded a certificate ceremony.
The students killed were ages 6 to 8, Police Deputy Inspector General Qazi Wahid told reporters at the scene.
Wahid said the bomb was apparently hidden in a bicycle parked outside the school. The bike was decorated with flower garlands, which apparently fooled security guards into believing it was part of the promotion ceremony.
Nearly 50 people were injured in the blast, and police said the toll could have been higher if the bomb-laden bike had been wheeled inside the school.
The Pakistan Daily Times said there were no immediate claims of responsibility for the bombing. Madrassas have not been the targets of any threats in the city despite an ongoing crime problem, a city police official told the newspaper.