They're not necessary in school.
In the classroom in particular they should be confiscated if out, since they're a distraction to both the person with the phone and those around.
Outside of the classroom they're not necessary either.
I think it should depend on an age thing on if they should be allowed in schools or not. I don't see why a young teenager or pre-teen should need a phone on them.
And having access to an internet-connected smartphone in school could be open to abuse.
they have no right to take your property. make you sit in the hall or go to the office, fine, but they should not be able to take your property.
They give it back. My schools always gave confiscated items back to students at the end of the day.
It's up to you the student to not bring it in anyway.