"Some unknown people entered a girls' school and poisoned the classes, when the girls come to classes they got poisoned," a police spokesperson said
Herat commander Mawlawi Ansari told Reuters that nine people had been injured. The cause of the blast was not clear.
A health official in Herat, who asked not to be named, said an explosion hit a small van used for public transport just after 1800 local time and that three of the injured were in serious condition.
Since the Taliban took over in August, a series of blasts and attacks, some claimed by Islamic State, have taken place across Afghanistan.
The attacks have heightened the new administration's security challenges as the country spirals into an economic crisis.