But since there are cell phones. Why should parents be compelled to reach their kids via a school administrator when they could just call their kid?
Because, legally, the kid is under the guardianship of the school. The school needs to be aware of where the kid is at all times, especially if they have to leave school early. Also, who says it's just parents who have access to the kid's cell phone? Other students who are home sick, etc, could call.
I don't see how stealing someone's possessions is justifiable.
It's not stealing. It's confiscating. They get it back. And it's justifiable because it's against the school rules. They sign a handbook at the beginning of the year. It lists the rules. Their signature shows that they agree to abide by the rules. One of the disciplines for infracture of the rules is confiscation. If they read the contract, they know this.
Kind of like how the police can "steal" (confiscate) your crystal meth lab.
Why not just ask the person to leave class to take the call or ignore the call?
Leave class? Then they miss material. Ignore the call? Then it rings and rings. Anytime you distract the class, no matter how minor the distraction, it detracts from their learning.
30 kids in the class. That means at least 40 different parents who could call (many are divorced and remarried). Between after school sports pick up, bus delays, kids staying at other kids houses, etc, you could have 3-4 phones ringing per period.
Heck, I even keep my landline on "silent" mode during class. If there's a real emergency, someone will knock on the door. (It's never happened.)