TLIK: It isn’t ???? since the parties effectively agreed to allow that condition beforehand. And there is nothing wrong with contractually agreeing to that condition. It would be no different than if you and I contractually agreed that if the Yankees win the world series your wristwatch becomes mine. Then my taking your wristwatch after the Yankees win is not ????.
A second and additional point allowing it is that the teacher is essentially saying “if you wish us to continue allowing you to be a student of this school, the cost is for you to surrender that cell phone”. The student can choose to not pay the price of surrendering the cell phone, keep it, and leave the school. It is the schools legal, moral and disciplinary right to enforce that condition.