"All the victims are women teachers and students," according to a police official, Mir Zubair Mehmood, in Quetta. The blast shattered the windows of offices and classrooms inside the school.
The bus was parked at Sardar Bahadur Khan Women's University in Quetta, he said.
The bus was run by the university, which is on the outskirts of Quetta, the capital of Balochistan province.
There was no immediate claim of responsibility.