Not true. No debate is allowed in the school perhaps, but parents aren't helpless. If your child is being taught something in school, and you want them to see the argument from a different standpoint, teach them yourself. Don't rely on schools to be the only source of education for your children.
I can't count the number of times I'd come home from school, be asked "what did you learn today", relate the stories of that days lessons, and be asked "Hmm... and what about...?", which led to alternate ways of looking at nearly everything. Some were simply from the devil's advocate view, just to improve my rational and critical thinking skills and teach me to defend what I believe to be true. Others were intended to make me see there is more than one way to approach nearly every issue.
Point is, parents have more influence over their children than they believe. Disagree with what the school teaches? Fine. Tell your students that's what the school thinks, but challenge them to see things from another viewpoint as well.