Generally, schools are a public space (unless you’re in private school). You have no right to privacy in public, nor do you have copyright on the photo. You can request they remove it here: https://help.instagram.com/contact/504521742987441?helpref=page_content but there is a good chance they won’t.
Edit: you live in Italy which has a specific law that photos of children cannot be published w/o permission. Just mention that and they’ll likely remove it.
I’ve seen this mentioned so much, I’m gonna have to check it out.