They did try to evacuate. The building was hit while they were doing so:

> Hundreds of students are believed to have been inside the building by the time teachers and administrators received the news that bombs had begun falling on Tehran around 9:40 a.m.

> Teachers and administrators thought it prudent to send the children home. They called parents on landline phones, summoning them for an early pickup, two people told the AP. A recently released report by Airwars, a London-based independent group that tracks recent conflicts, also found that parents were called to pick up their children.

> At 10:15 a.m., Iran’s state media sent out an advisory, closing schools across the country.

- https://apnews.com/article/iran-school-strike-baluch-trump-2...

> if you think that America would leave children in that school for longer than 10 minutes after a war was launched

Dude, the US doesn't even protect American schoolchildren from other Americans, let alone a foreign power: https://en.wikipedia.org/wiki/Uvalde_school_shooting