Public Freakout Videos - Principal Refuses To Give Back His Daughters IPhone 6 & Offers Him A Flip Phone Instead!

There's a point where the teachers need to stop being the parent and let the actual parents have that role back.
During school hours, they're in charge
But after school hours, that's pushing it

"In loco parentis," I believe it's called. Schools DO technically have the right to do this for the same reason that you as a parent have the right to take your child's phone away. By enrolling in the school, you agree to their rules and policies. If confiscating your child's phone is in their policy, they are legally allowed to do it. It is not theft because in order to prove theft, you must prove that the school intended to keep the phone forever.
That is the legal aspect. Now the personal aspect:
Although schools do have a right to do this, I think "a few weeks" is dickish. Especially when you have the parent there asking for it. Confiscating the phone for the day is fine, and if there are repeated mishaps, you should confer with the parents about it, or if they don't help, take it upon yourself. I mean, if it gets to that point, hey, you have a good reason to confiscate it for an extended period of time. But if that is not the case, the phone should be returned by the end of the day, or by parent pickup.

OH! However, if by "secure the phone" she means they need to go through and check the phone, no. Absolutely against the 4th amendment, and they cannot do that. And if they do anyway, and bring up any sort of charges based on that evidence, it cannot be used in a court of law.