Speaking on the sidelines of a ceremony to mark the National Tree Planting Day, Ayatollah Khamenei described the poisonings as a heinous and unforgivable crime.
"If the poisoning of students are proven, perpetrators should face maximum punishment with no pardon," he said.
Earlier, President Ebrahim Raisi issued strict orders for pursuing the issue of poisonings in schools.
Raisi said the poisonings are a new plot by Iran's enemies as he described them as an inhumane crime.
Ayatollah Khamenei on Monday planted a sapling in the premises of his office on Monday morning.
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