The incident took place in Herat city when the girls were at school and were busy studying, said officials.
The girls and their teachers have been taken to a nearby hospital for treatment, officials added.
“So far 16 school girls and four teachers have been transferred to hospital for treatment,” a spokesman for the Herat Regional Hospital, Mohammad Rafiq Sherzai said.
He did not provide details on their condition but said that more patients were expected.
He said: “The transferring of the poisoned girls [to hospital] still continues.”
The poisoning of school pupils, especially at girl’s schools, has increased in recent months – which has sparked concern among many families in Afghanistan.
With today’s incident adding on, this was four of its kind within a few days in west province of Herat.