Tehran, Kermanshah, Rasht, Orumieh, Tabriz and Hamedan are among the provincial capitals where poisonings happened.
The poisonings cause dizziness, sore throat and headache, sending the students to hospitals.
Iranian government officials have said they are investigating the issue.
Meanwhile, parents of school girls are criticizing officials and say they are responding to the incidents too slowly.
Iranian President Ebrahim Raisi earlier instructed the interior minister to get to the bottom of the matter as soon as possible.