The Debatabase is a nifty resource for middle and high school students examining controversial issues. This may be helpful for introducing older students to critical thinking as well. Most students (hey, even adults) are poorly trained at examining their own assumptions, bias, and arguments. So it helps to have pro and con positions written on for both sides, so they can really be understood. Many students may have to read many before they begin to recognize the pattern of providing support for statements and format for communicating their ideas.
For younger children, check out the critical thinking article below about Fairy Tale ethics. Elaine Lindy looks at the 'trickery' of Puss n Boots and 'stealing' in Jack and the Beanstalk.
Finally, check out the classical critical thinking approach that employs 'progymnasmata': "A set of rudimentary exercises intended to prepare students of rhetoric for the creation and performance of complete practice orations (gymnasmata or declamations)." The progymnasmata begins by rewriting Aesop's fables as a preparation for the systematic thinking, writing, and presentation of rhetoric. The last link below has some sample original and rewritten Aesop's fables.
Fairy Tales & Ethics