This popped up in one of my searches and was too good not to pin! The more students that have cell phones, the more i think this could come in handy. Plus it is a funny way to talk about a real issue of having phones in class.
I would reword it to say: If you didn't EARN the grade you wanted, its likely you didn't do the work you're capable of". My new favorite poster (which is FREE and ready-to-print from Teachers-Pay-Teachers)