Actually, I think they do try to teach it in schools, albeit often indirectly and, of course, not always as well as perhaps we would all like. Here are a few ways:
In English literacy classes children are taught to consider how word choices change meaning, to understand what indirect messages are being used by word choices, tone etc.
English comprehension is as much about learning to recognise the facts in a piece of writing as it is about learning to think about the underlying meanings, suggestions or implications.
In history, children are (supposedly) being taught about different types of sources, how to assess them, how to check or cross check them etc. (I appreciate this is done at a fairly basic level in most cases).
We see children being encouraged to use news writing as a type of writing they should learn as part of broader themes - from my Year 5 daughter being told to write a news story about their school residential to DS year 9 class writing a news article about the death of King Henry 8.
Even in classes like IT and computer skills now where children are being taught about internet safety - part of those classes is attempting to make them understand where information is coming from and how to assess it.
Clearly however, these tools are not working well enough, based on the cluelessness of a lot of people on threads like this.