Minors cannot leave the ship unless they are accompanied by an adult on their booking. I know this from first hand experience when our son got ahead of us in the line leaving the ship. When they scanned his card they got an alert screen and the guard wouldn't let him exit until we got up to the security station.
You know what I'd do if you really wanted to let your daughter go on the cruise? I'd go too. Either go as a school chaperone or make your own individual booking.