Know the school rules.
Together with your child, read up on any existing school policies regarding cell phones. If your child breaks rules and loses in-school phone privileges, don't butt in. Like you, the school administration should be permitted to enforce its own regulations.
Mean what you said.
Rules do no good if they are constantly flouted without consequence. If your child has trouble staying under allotted minutes, switch to a prepaid model, or consider an app like "My Mobile Watchdog," which cuts off service at designated times. And don't hesitate to "ground" your child from cell privileges when rules are broken.
Be available to help.
If your child misuses a cell phone and lands in hot water or becomes the target of bullying from peers, he or she should feel comfortable coming to you for help. A quarter of teens report having received harassing text messages and phone calls, according to Pew Research Center data, and cyberbullying can begin as early as elementary school.
Sources:
National Consumers League:
http://www.nclnet.org/tweens_and_cell_phones_a_guide_for_responsible_use
Pew Research Center:
http://www.pewinternet.org/2010/04/20/teens-and-mobile-phones/