I would try a different tack from what's suggested above: Announce the problem at the next general meeting, in the New Business portion of the agenda. Give it a month to see if it improves. If not, check your bylaws for what provisions exist to remove a nonperforming officer from office, and announce appropriate action at the following meeting.

For instance, PTO Today's sample bylaws say, "Officers can be removed from
office with or without cause by a two-thirds vote of those present
(assuming a quorum) at a regular meeting where previous notice has
been given." So give notice (you might have to get this published in the school or PTO newsletter or print and distribute it yourself to satisfy this requirement), and then hold a vote at the next meeting, and recruit as many people as you can to come and vote.

If it gets that far, your motion to remove them from office might not carry -- or it might, and you might find yourself in charge of a better group -- or of nobody. So there are risks, but there are also risks to the PTO and the school to allow renegades to run the PTO without regard for the rules.

According to Robert Rules of Order, 10th edition, page 255, line 13 through 27 (paraphrased):

The basic right of the individual member cannot be suspended, even by unanimous consent or unanimous vote.....so as to deny any particular member the right to attend meetings, make motions, speak in debate, and vote, which are basic rights that may be curtailed only through disciplinary proceedings.

HOWEVER, it sounds like your PTO is run by a renegade posse of two who give no credence to the parliamentary process.

So, the question to ask yourself is if it's really worth the aggravation to fight this clique in order to then experience the on-going misery of trying to effectively get them to work with you? In this kind of situation, you could find your reputation taking a beating when you can't perform your duties due to the roadblocks they will keep throwing in your path.

Since the principal is a limp noodle, unless there is a higher authority (like a state PTO of which your school's PTO is a member chapter) to which you could appeal, your best bet may well be to tender your resignation and leave with your good name intact.

I am a duly elected member of my board of directors. The current President and Vice President have decided not to include me in the meetings of the Board. They do not inform me of when Board meetings will be held.

They have taken away my ability to serve by not informing me of the meetings.

Are they allowed to do this, and what is my remedy to get them to include me in future meetings? I have addressed this with the Principal and he has done nothing about it. If it matters, our PTO is governed by Robert's Rules.