Our son had his iPhone stolen from him at school. He was in 6th grade and put his bag down to play ball when someone must have walked up and rummaged through his backpack. Once he realized it was stolen he called me from his friend's phone. I jumped on Find My and had it down to a house. They turned it off as soon as I pinged and sent a message to it. So, I called the principal and told her, "Could you ask the student who lives at 555 Friendly Lane in North Beach to return my iPhone or I'm going to call the police." The reaction she gave kind of let me know she knew who they were. Kiddo had the phone back in the morning and got a scolding from me and the principal about guarding his things.
If the other kid didn't return it I don't know what we could have done. But I think mentioning the police were involved made him, probably 10-12yo, feel he was over his head. An adult would probably tell us to fuck off.