The #1 thing you can do is to teach your child to tell you EVERYTHING!!! When your child tells you something, be NON- JUDGEMENTAL...you want to keep the lines of communication open. This gives you the chance to refute what is being taught in school and assert your position as the primary educator of your child.
For example, my 4th grade Daughter brought home an exercise in Cause and Effect that conveniently used Global Warming to demonstrate the relationship. We had fun and she learned the difference between real science backed by real evidence. She also learned that not ALL scientists believe that Global Warming is caused by Man.