Amid a plethora of back-to-school reminders, Missouri public schools are making a special effort to inform parents about a new state law banning cell phones during the school day. The law, signed by Gov. Mike Kehoe in July, requires school districts to set a policy barring students from using mobile devices except in emergencies or when instructed by a teacher. And with most students returning to classrooms this week, districts have been warning parents about the new expectations.