Actually our PTA board meeting are for board members only. This consists of President, 3 VPS, Secratary, Treasurer, school Prinicipal and any ongoing committee chairs. These are not advertised to the general public. Should any one other than board not want to attend they would most likely be turned away - that has not happened to date.
However - no voting can happen at these meetings. The board cannot vote to do something. That has to be done at the General meeting. We normally meet for about an hour two weeks before our general meeting (we have to do it this ways as our bylaws state this).
These are generally meetings where we set the agenda. review what has gone on at the district level, talk with the Prinicpal about current school issues/needs - from that info we focus on where to go and what/how to present all the information.
As for the school that has board meetings every month but only one membership meeting? That seems off to me. I cannot spend money or make a PTA commitment without the buy in the of the PTA as a whole.
Those things REALLY surprise me. Certainly we have certain things that have to go before the general membership, but why have a board meeting if no decisions can be made there?