When the wife and I decided to give our kids cell phones I told them very clearly that the phone was theirs, a reward for good grades. I followed that up with the kicker. The service (read, monthly phone bill) belongs to mom and dad and would be turned it off if rules were broken or grades began to slip. Found out very quickly that it was the daily "phone off" hours that were trampled on. Texting instead of doing homework, staying up until o-dark hundred calling and texting friends or texting during class were the biggest problems. Both kids realized very quickly just how fast the phone service could be turned off and the problem ceased.
For any parents thinking of letting their kids have phones, if you are going to have any rules at all I would advise setting clear times when the phone is to be turned off. We stay pretty active in our kids school life and without fail all of our kids teachers will tell us that kids, despite school policy, will spend more time texting their friends then paying attention in the classroom. We also found, rather quickly, that teenagers staying up into the a.m. texting instead of sleeping are some of the grumpiest little human beings on the planet...for those that have teenagers, you can relate. Okay, off the soap box.
Cheers