What you want to avoid are:

* Materials that encourage the student to challenge biblical authorities. Typically, critical thinking materials urge children to ignore the teaching of parents and church authorities. They call this "thinking for yourself." But this is not real thinking, simply knee-jerk rebellion.

* Materials that encourage general confusion and skepticism. Eighth grade is not the time or place to ponder the nature of reality.

* Materials that dwell at length on a particular view of the future -- typically, one in which Christianity is irrelevant and a one-world government will solve all human problems.

* Materials that introduce "closed" dilemmas. The most famous is probably the story of the hapless husband who doesn't have money for needed medicine for his dying wife. Is it OK for him to rob the pharmacy to get her medicine? When Christian students try to point out in their public school classrooms that the husband can always pray, and that God can either heal the wife directly or bring in unexpected money, the teacher tells them that this solution is impermissible. Only choices that leave God out of the picture are OK. But in real life God is always in the picture, so we never have to face this kind of phony dilemma.