Speaking in a ceremony to mark the National Tree-Planting Day on Monday, Ayatollah Khamenei said if the poisonings of students are established, the perpetrators must be handed the severest punishment.
The leader stressed that “no amnesty” will be offered to such people.
Authorities have so far confirmed dozens of attacks on girls’ schools in the capital Tehran and several other cities across the nation in recent months.
The attacks have sent an unspecified number of students to hospitals for treatment.
Iranian President Ebrahim Raisi has blamed the country’s enemies for the attacks.