TexasCHLforum.com

Not a legal opinion, of course, but I do not think that school district administration buildings count as "schools" with regards to CHL laws, as there is no instruction activity, and usually no school kids taking classes in the building. That's with regards to larger ISD's like Frisco, Fort Worth, Northwest, etc. For a very small district that might house their admin folks inside a school building, that'd be different. But, not under the circumstances that you're describing.

But, that said, you'd still be prevented from carrying since the ISD is most certainly a governmental agency, if you were there at the school board meeting, that would qualify as a governmental meeting.
At other times, I'd say that it couldn't be posted with 30.06 (well, it COULD, but it wouldn't mean anything) since it's a property owned by a governmental agency.