There are no quick and easy ways to remove one board member much less an entire board. I take it that your PTO's by-laws contain no term limits? How much authority per the by-laws is the principal or school given over the board's activities?
I suggest you post your situation with the pertinent details on the Robert's Rules of Order forum for expert assistance. Here is the link:
http://www.robertsrules.com/cp/Scripts/ASP/forum/