"Chemical compounds" were used to deliberately poison girls at schools in Iran, a senior health official told the semi-official Fars news agency.
“Certain individuals sought the closure of all schools, especially girls’ schools,” Younes Panahi, Iranian deputy health minister, said on Sunday.
The poisonings have led to dozens of girls needing hospital treatment.
They come months after widespread protests against Iran’s leadership erupted over the death in police custody of Iranian-Kurdish woman Mahsa Amini, 22.
The authorities have responded with deadly force and at least four protesters have been executed.
Thousands of people remain in prison for taking part in demonstrations and rights groups say more than 500 people have been killed by security troops.
The poison attacks have all been against girls’ schools and were first reported in November at a secondary school in the city of Qom.
At least 14 schools have been hit in four cities, including the north-western city of Ardebil, the capital Tehran and the western city of Borujerd, newspaper Etemad reported.
Videos shared by state news agencies and activists on social media this month have shown angry parents confronting education officials and teachers demanding a response to the incidents.
Ms Amini went into a coma shortly after being arrested for wearing her hijab "improperly".
The subsequent protests, which were initially led by women and young people, broadened into a nationwide rebuke against the ruling religious system.
Qom, where the first poisoning was reported, is about 160km south of Tehran.
It is a deeply conservative and religious city that is home to Iran’s clergy and theological seminaries, where most of the country’s leaders and presidents have studied.
The poisonings also come at a time when the Taliban in neighbouring Afghanistan continue to keep schools and universities closed to girls and women, effectively banning them from education.