I teach middle school and each year I start the year by stating that there are such things as stupid questions. I state that if they are asking questions about what we are learning, they aren't stupid questions. But if a student asks a question about a direction or something that I have just told the class (nonacademic) because they weren't listening, that is a stupid question and it will be ignored. This saves me a lot of time dealing with kids who don't pay attention and ask questions about something that I have already told the class.