Texas doesn't use the term "custody", and instead speaks of managing conservatorship. If you and your ex- are joint managing conservators, and the custody order does not provide otherwise, you are expected to work out this type of issue between yourselves. If you cannot, you can petition the court to make the decision for you based upon what the court finds to be the child's best interest.
You refer to joint custody, then later to a "custodial parent". Without your being clear about the facts, we cannot be certain that information we provide will be on-point.
If in fact the child has primary domicile with one parent during the school week, and the other parent lives in a different school district, and all else being equal, a court would likely order that the child attend the neighborhood school of the primary custodian. Beyond that, the court would have to look at the more detailed facts.

