My friend, fellow skeptic, and fellow Skeptologist Brian Dunning has put together a video on how to think critically. It's called Here Be Dragons, and it's a pretty good primer into how to think.
It's about 40 minutes long, and free to use (with some caveats; see the site). I think this would do well in a classroom. Any teachers out there? I know it's too late for most school sessions, but you can download the movie (and a high-res version too) and keep it handy for the next year.