Before I went to the school board I would attempt to schedule a sit down, one on one meeting with the Principal (after school hours of course

) to discuss the issue. Clearly stating that you feel there is more to this then what has been stated. Try to find out the real reasons (if there are any), explain what the students will miss out on if the PTO is banned and also remind her of the many reports and statistics that clearly state that kids do better in school when parents are involved.
If that doesn't change the decision or if more valid reasons can't be given then I would move on to the school board.
Almost makes me wonder if there isn't something else going on though that the principal is trying to protect the kids from in some fashion but is limited on her abilities to do so.... to protect from potentially one or two people she has made a sweeping policy.