Gosh it's many years back now, but as someone starts to notice you ask them what you think you'll draw next. It ends up with drawing the mouth and hopefully by the time you do that you can draw a 'smiley face' because the class is now prepared for the lesson. (So you have to add lots of extra detail until you've got that.) We spent a bit of time discussing why the face was significant and how important it was that they were prepared to learn in the lessons for their own benefit. Here's a link to an article from the Guardian, bit dated now but suggestions and thoughts are relevant. Read a few books, There used to be one called, Getting the B******* to Behave' by Sue Cowley which I read, but there must be loads more out now.