The ****?
That's not their phone to keep. It's one thing to take it from someone who's using it during class and then eventually give it back to them, but it's another thing to not give it back at all. That's stealing.
edit: Oh I see she said they'll give it back to them in a few weeks.

It's still just as ridiculous. I remember at my school, if you were caught texting or whatever they'd take your phone away and you'd have to pick it up from the principal at the end of the day.
But even then, if a parent comes down to pick it up, you normally should be required to give it to them since they're an adult and it's their property.