Debbie Drown, Educator, Principal, says:

Being involved in your son's schooling is probably one of the most important things that you can do.   But, here are a few tips for making it work for everyone. 

1) Don't base opinions about teachers on what other people say.  Get to know your child's teacher before making a request for a particular teacher or asking to have your child moved.  If you decide to ask for a particular teacher be sure to approach the principal with factual information to support your request.  
2) Always have any conversations with your spouse or neighbors about your child's teacher out of earshot of the children.  
3) As for volunteering, I always suggest that parents volunteer to work anywhere in the school that they are needed, not just in their child's classroom(s).  Children sometimes behave differently if their parent is working in their classroom. 
4) Always remember the confidentiality rule when grading other children's papers or observing other neighborhood children while in school.