**So if I have this correct... a quorum for a board meeting would be a majority of board members in attendance, or 2/3, whatever number we decide on.**
Almost. Not board members in attendance, but filled board positions. There's a difference. If 4 members out of the 9 filled board positions attend quorum doesn't calculate off of the 4 -- it calculates off of 9. Also, be certain to use the words "filled board positions" in your by-laws so that your quorum calculation won't be put in jeopardy if the board unexpectedly has open positions. Other than that, yes, you can select whatever number for qorum the board would prefer: simple majority, 2/3rds, 3/4ths, whatever.
**It's the quorum for a general meeting I'm struggling with. A majority of what?**
Robert's Rules of order on determining quorum in a mass meeting states that it is simply the number of persons present at the time, since they constitute the entire membership at that time (10th edition, pg. 334, line 20). In organizations such as many churches or some societies in which there are no required or effective annual dues and the register of members is not generally reliable as a list of hte bona-fide members, the quorum at any regular or properly called meeting consists of those who attend (10th edition, pg. 335, line 1-5).
To accomplish their work, voluntary societies that have an enrolled membership generally need a provision in their bylaws establishing a relatively small quorum -- considerably less than a majority of all members (10th edition, pg. 335, line 15). The quorum should be as large a number of members as can reasonably be depended on to be present at any meeting, except in very bad weather or other exceptionally unfavorable conditions (10th edition, pg. 335, line 30).
SO........ this is a double-edged sword. It relieves the board of having to beat themselves up trying to get a huge crowd at general meetings. However, it also leads to the danger of allowing a very, very small percentage of school families to vote in decisions that will affect everyone.
Hope this helps.