Slang isn't a completely different language. People adjust their way of speaking depending on who they are with and as zetamcfc says, it's a violation of freedom of expression. Of course, if you are defining the motion, you could be clever about it and say 'This house believes that slang should be banned in schools, with the exception of conversations outside the classroom' - of course you can't really stop people from using slang, teachers couldn't go around giving detentions for uses of slang as they probably use it themselves.