Students use cellphones in school to text, bully, plan fights, and sometimes learn

Here are Common Sense Media's guidelines for cellphone carrying kids. (Fotolia)

Once banned by most US school districts, cellphones now are as much a part of the class day for many students as No. 2 lead pencils were for their parents. 

Teachers put them to use as metronomes in music classes, for instance, and give quizzes through a popular app called Kahoot! Parents text their kids to ask when they'll be home or remind them to walk the dog when they get there.