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)
DIY stamps - looks pretty easy. use a wine cork and I would use a piece of the craft foam that is easy to cut. Then glue it onto the cork. This one it looks like they use the precut wood shapes at the craft store.