I know lots of schools have the rule. I would tell my kids to ignore the rule and to keep their phones with them, set to either silent or vibrate. If I needed to reach them (or vice versa), that overrides such rule.
I don't mean to turn this into a cell-phone rant. They are a great convenience, and in extreme situations can be a life-saver. But ...
in the past week alone, I have heard cell phones ring or vibrate very loudly during a classical concert, a baptism at church, a college seminar room, a banquet speech (it was a Rudy Guiliani moment), and a job interview (yes, the candidate's phone!). Many people who otherwise have fine character and admirable traits have lost all sense of propriety and courtesy when it comes to cell-phone use in the presence of others.