Media literacy and how to validate sources. Unfortunately, the second part was primarily taught in college when I was still in school.

Critical thinking is very difficult to teach. Its so much easier for people to just accept whatever confirms their current preconceived notion. It also requires that the person is both open to learning new things and that they are open to the idea that they may be wrong, misinformed, or not know everything.

So many people are simply over confident about their own knowledge.