I would say that you just need to get your parents organized and then ask for volunteers to form a committee to study the advantages/disadvantages of organizing on your own or joining the PTA. Our group looked at what the PTA had to offer and decided whether we would actually take advantage of those offerings and if it would be worth the money for our group to become a part of that organization. We decided to be independent and organized ourselves as our own non-profit group. It was a personal perference of the parents in our group-we felt we could better tailor our group to the needs of our individual school by being our own independent group, and that we wouldn't take advantage of the programs offered by the PTA, so it would not be worth the money to join PTA.
