Keep repeating it: there are no bad students. This attitude will take you far. Sometimes your students will make suboptimal decisions, and sometimes their attention will wander. What gives? You had an engaging lesson and you've done lots to motivate this class! These things happen. And you can't ignore them. The prepared teacher will have a variety of proactive moves to maintain attention and set students up for success, as well as a tool belt of ways to consistently react when things go wrong.