Well, we here at the board hear these types of issues all of the time and usually have to inquire about a few things.
1. Do you know if your group is an independant organization?
Some groups, whether called a PTO or not, are under the umbrella of the school, meaning that they are effectively controlled by the Principal. In organizations such as this, I think of the group as a fundraising committee that does all of the work to raise funds, but in the end the Principal gets to decide on how they are spent. As you can imagine, I am not a big fan.
Most groups like ours are independant 501(C)3, meaning they have filed the appropriate paperwork to become registered as independant. Being independant gives your group the control, most often.
2. Does your group have Bylaws?
As LUVMYKIDs mentioned, this really comes down to how your group is organized. That is usually defined within a set of Bylaws. Sometimes they are vague, but often they will be quite specific, especially when it comes to the voting process, executive committee elligibility, etc.
If you have Bylaws your President should be following them. If not then someone should be talking with the President to inquire as to why the Bylaws are not being followed.
It's very possible that your group's President (assuming you are not in that role) is thinking the same things you are, but just doesn't know how to run the group properly, or possibly is nervous of confronting the Principal.
Most groups have it indicated in the Bylaws that elections are to held every year at a certain time. It may not have a specific voting method speicifed, but the general jist should be there.
IMO, it is the Presidents job to ensure that a school's Principal isn't overstepping their place. The President needs to take back control. Of course if your answer to question #1 is that your group is under the school, well then you can start thinking about trying to push becoming independant.
Good luck,
PresidentJim