One girl was reportedly injured in the security raid, according to an activist monitoring the situation.
Protests — which have led to over 200 deaths in a harsh government crackdown, have endured since September 16, with almost nightly demonstrations in multiple cities.
Monday’s violence was apparently triggered by authorities wanting to check the students’ phones as part of the crackdown. A teaching union was due to end a two day strike today, in protest against government violence that has led to the deaths of 23 children, but the security raid on the school was unrelated to the strike.
Nationwide demonstrations began following the death of Kurdish Iranian woman Mahsa Amini, 22, who died following a stay in hospital. Demonstrators say she had been severely beaten while detained by morality police, who had arrested her for wearing “improper attire”.
Iran's Ministry of Education said several students were treated by emergency services for a drop in blood pressure, but denied security forces had entered the school.
Videos circulating on social media showed heavily armed security forces outside the school.
One clip showed them on motorbikes and firing at least one tear gas canister. Reuters could not independently verify the authenticity of the footage.
Women have played a prominent part in the protests, removing and burning veils. The deaths of several teenage girls, reportedly killed during protests, have fuelled more anger.
Demonstrations continued on Monday, with a rally at a university in the western city of Hamedan, and students shouting down a government spokesman who visited a Tehran university, according to students and rights groups.
The education ministry said there was a clash at the Tehran high school between staff, students and parents after the school principal insisted on checking the girls' phones.
The widely followed activist Twitter account Tasvir1500 said uniformed forces attacked the school and at least one girl was wounded, but city police denied the account.
“After news of a conflict near a high school … police were dispatched to the area and investigated the issue which turned out to be a fight between a number of thugs,” Tehran police said. Agitators were identified and arrested, police said.