Iranian officials acknowledged on Sunday that girls at more than 50 schools were victims of poisonings.
Fear about the poisonings has spread among parents after months of unrest in the country.
The poisonings began in November in the holy city of Qom and it remains unclear who or what is responsible.
Reports now suggest schools across 21 of Iran's 30 provinces have had suspected cases, with girls' schools the site of nearly all incidents.
The attacks have raised fears that other girls could be poisoned, apparently just for going to school.
Education for girls has never been challenged at any time since the 1979 Islamic Revolution.
Iran has been calling on the Taliban in neighbouring Afghanistan to allow girls and women return to school and universities.
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Interior Minister Ahmad Vahidi on Saturday said that investigators recovered “suspicious samples” in the course of their investigations into the incidents, the state-run Irna news agency said.
Mr Vahidi called for calm among the public, while also accusing the “enemy’s media terrorism” of inciting more panic over the poisonings.
But it was not until the poisonings received international media attention that hard-line President Ebrahim Raisi announced an investigation into the incidents on Wednesday.
On Sunday, Mr Raisi told the Cabinet, following a report read by Intelligence Minister Ismail Khatib, that the cause of the poisonings must be uncovered and confronted.
He described the attacks as a “crime against humanity for creating anxiety among student and parents".
Mr Vahidi said at least 52 schools had been affected by poisonings.
Iranian media reports have put the number of schools at more than 60. At least one boy's school has reportedly been affected.
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Videos of upset parents and schoolgirls in emergency rooms with intravenous drips in their arms have flooded social media.
Making sense of the crisis remains challenging, given that nearly 100 journalists have been detained by Iran since the start of protests in September over the death of Mahsa Amini, 22.
Ms Amini had been detained by the country's morality police and later died.
The security force crackdown on those protests has led to at least 530 people killed and 19,700 detained, according to Human Rights Activists in Iran.
The children affected in the poisonings reportedly complained of headaches, heart palpitations, or feeling lethargic or unable to move.
Some described smelling tangerines, chlorine or cleaning agents.
Reports suggest at least 400 schoolchildren have fallen ill since November. Mr Vahidi said two girls remain in hospital because of underlying chronic conditions.
As more attacks were reported on Sunday, videos were posted on social media showing children complaining about pain in the legs and abdomen, and dizziness.
State media have mainly referred to these as “hysteric reactions".
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Since the outbreak, no one was reported in critical condition and there have been no reports of deaths.
Attacks on women have happened in the past in Iran, most recently with a wave of acid attacks in 2014 around the city of Isfahan, at the time believed to have been carried out by hard-liners attacking women for how they dressed.
Speculation in Iran's tightly controlled state media has focused on the possibility of exile groups or foreign powers being behind the poisonings.
That was also repeatedly claimed during the recent protests, without evidence.
In recent days, Germany's Foreign Minister Annalena Baerbock, a White House official and others have called on Iran to do more to protect schoolgirls, which Iran's Foreign Ministry has dismissed as “crocodile tears".
But the US Commission on International Religious Freedom said Iran had “continued to tolerate attacks against women and girls for months” amid the recent protests.
"These poisonings are occurring in an environment where Iranian officials have impunity for the harassment, assault, rape, torture and execution of women peacefully asserting their freedom of religion or belief,” the commission's Sharon Kleinbaum said.
Suspicion in Iran over the poisonings has fallen on hard-liners.
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Iranian journalists, including Jamileh Kadivar, a prominent former reformist politician at Tehran’s Ettelaat newspaper, have quoted a supposed communique from a group calling itself Fidayeen Velayat.
It reportedly said that girls' education "is considered forbidden" and threatened to “spread the poisoning of girls throughout Iran” if girls’ schools remain open.
Iranian officials have not acknowledged any group called Fidayeen Velayat, which roughly translates to English as “Devotees of the Guardianship.”
But Ms Kadivar’s mention of the threat in print comes as she remains influential within Iranian politics and has ties to its theocratic ruling class.
The head of the Ettelaat newspaper also is appointed by Supreme Leader Ayatollah Ali Khamenei.
Ms Kadivar wrote Saturday that another possibility is “mass hysteria."
There have been previous cases of this over the last decades, most recently in Afghanistan from 2009 through 2012.
Then, the World Health Organisation wrote about so-called “mass psychogenic illnesses” affecting hundreds of girls in schools across the country.
"Reports of stench smells preceding the appearance of symptoms have given credit to the theory of mass poisoning," WHO wrote at the time.
"However, investigations into the causes of these outbreaks have yielded no such evidence so far."
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Iran has not acknowledged asking the world health body for assistance in its investigation.
WHO did not immediately respond to a request for comment Sunday.
But Ms Kadivar also noted that hard-liners in Iranian governments in the past carried out “chain murders" of activists and others in the 1990s.
She also referred to the killings by Islamist vigilantes in 2002 in the city of Kerman, when one victim was stoned to death and others were tied up and thrown into a swimming pool, where they drowned.
Ms Kadivar described those vigilantes as members of the Basij, an all-volunteer force in Iran's paramilitary Islamic Revolutionary Guard Corps.
“The common denominator of all of them is their extreme thinking, intellectual stagnation and rigid religious view that allowed them to have committed such violent actions,” she wrote.