Ricardo, agreed. You mentioned girls school Dave, not telling you how to raise your kids, but it seems to me like this might be an option you'd wanna seriously consider. Taking away the phone isn't going to solve these problems, if they're as serious as they seem to be.
Well, if it's affecting their grades and other aspects of their lives negatively (which it cerainly seems to be), then that changes things entirely. If they were good kids who did well in school and such, then I would endorse being lenient and finding a compromise solution (as I had suggested)...
Why not try to come up with a solution that works for everyone? I think an extra phone line or a prepaid cell phone that they pay for is a great, fair idea; much better than trying to take away the phone entirely, which will only antagonize the daughters and make them hate you and do more to...