As a former teacher I can tell you that I seriously doubt it was taken for no reason, so let's 'fess up on that. Secondly, I also doubt he intends to keep it forever. He may be planning on taking it to your counselor or dean and having you and your parents in for a conference about why it was taken, after which it probably will be returned to you or your parents. Or he might be calling your parents and asking them to pick it up (& he will probably explain to them why it was taken). Either way you will almost certainly get it back eventually.
It must be school policy not to have them in class. Why don't you leave it in your locker or turn it off and leave it in your book bag? If she took it, that leads me to believe that you were using it in her class. Do you really see nothing wrong with that?
She does have to give it back eventually (unless you signed anything at the beginning of the year about teachers taking away your phones), but she can keep it for a day, or however long your school allows. To find how long this is, talk to your principle about it.