It sounds as though your best bet may very well be to go above or around your principal and go through the district. NCLB requires schools to implement a parent involvement policy, and districts that receive Title I funds can lose them if they don't. A really good explanation for that is located at
NCLB Action Briefs: Parental Involvement
. This doesn't guarantee that you can get a PTO started, but it should assist you in stopping the principal from discouraging parents from volunteering.
It isn't always a popular opinion, but in a situation like this it could prove beneficial to consider the PTA route instead of the PTO route, simply because the state and/or national PTA organizations may be able to assist you when you come up against this type of resistance. The national PTA has a page that tells who to contact for information about starting a PTA (
Why Your School Needs a PTA | PTA
). This isn't to say that you can't manage to start a PTO instead, just that if you meet resistance at the district level at solving the problem, they may be in a position to help.
All of that being said, it would definitely be very difficult to run a PTO in a school if the principal absolutely won't cooperate and no one at the district level will make her. You would need the ability to have fundraisers, family nights, etc., and the cooperation of the school to let you spend the money that you raise on things that the kids need. Ideally, your principal's boss would be willing to meet with you and her privately and then with the two of you together to set up a plan of cooperation so that you could function.
Have you tried asking your principal point blank why she is opposed to having parents involved at the school or having a PTO to support the school? Sometimes principals have misconceptions about what a PTO is and what they do that can make them resistant to the concept. If that's the case, clearing those things up could solve a lot of your problem.
What did the board say after they discovered that there wasn't a PTO in place? Were they willing to advise you in any way regarding the principal not allowing it?
HTH!
~Lisa