Yet another reason to keep your teen from talking you into a cell phone?Forget about passing notes in study hall; some teens are now using their cell phones to flirt and send nude pictures of themselves.The instant text, picture and video messages have become part of some teens’ courtship behavior, police and school officials said.The messages often spread quickly and sometimes find their way to public Web sites.Yes, yes, we know a lot of parents cite safety reasons for giving teens and younger cell phones,