It is generally thought that unless a "school" is accredited by the state of Texas then it is not recognized as a school in regards to carry. This leaves you with the traditional public schools Pre K -12 (including early childhood) and private schools that have accreditation through an organization that is recognized by TEA (i.e. South Associations of Colleges and Schools and others) and maybe a few trade schools that you can't carry into the premise.
While I don't think there has ever been a definitive statute or code found directly referencing the definition, there have been several citations from other statutes that seem to validate this view. You'd have to do a search to see the previous discussions.
Some churches house accredited private schools so the issue becomes even more clouded as to when is it a school and when is it a church or what part of the building specifically is considered the school or church. It must not be a pressing issue for the legislature as they have not addressed it since CHL was begun in Texas and, to my knowledge, has never been tested.
Of course, from watching the testimony for volunteer church security carry, the legislators may not even know there is a question as some on the committee were seemingly surprised volunteer carry was an issue in the first place.