“What about a gun?”
Your being silly... A cop should confiscate a gun. A teacher should not even try unless there is a threat involved.
In any case, for something like a phone, a student should be given the option of leaving with it or voluntarily turning it over.
You are the one being silly. You spoke in absolutes without any regards to the entire gambit of human actions and the surrounding events. Even a common pencil can be used as weapon.