Texas Family Code says that my wife is required to pay support until the child reaches the age of 18 or graduates high school, whichever is later.
That's not quite accurate. The applicable provision says that "[t]he court MAY order either or both parents to support a child in the manner specified by the order: (1) until the child is 18 years of age or until graduation from high school, whichever occurs later." Tex. Fam. Code section 154.001(a) (emphasis added).
Moreover, section 154.002(a) provides that "[t]he court may render an original support order, or modify an existing order, providing child support past the 18th birthday of the child to be paid only if the child is:
(A) under Chapter 25, Education Code, in an accredited secondary school in a program leading toward a high school diploma;
(B) under Section 130.008, Education Code, in courses for joint high school and junior college credit; or
(C) on a full-time basis in a private secondary school in a program leading toward a high school diploma; and
(2) complying with:
(A) the minimum attendance requirements of Subchapter C, Chapter 25, Education Code; or
(B) the minimum attendance requirements imposed by the school in which the child is enrolled, if the child is enrolled in a private secondary school."
Finally, section 154.006(a) provides that, "[u]nless otherwise agreed in writing or expressly provided in the order or as provided by Subsection (b), the child support order terminates on:
(1) the marriage of the child;
(2) the removal of the child's disabilities for general purposes [i.e., upon the child reaching the age of majority];
(3) the death of the child;
(4) a finding by a court that the child:
(A) is 18 years of age or older; and
(B) has failed to comply with the enrollment or attendance requirements described by Section 154.002(a);"
or a couple of other things not apparently relevant to your wife's situation.
Thus, it's not a matter of having the code clarified, it's a matter of what your wife's divorce decree or child support order says.
You can read all of the foregoing sections here: http://www.statutes.legis.state.tx.us/Docs/FA/htm/FA.154.htm#154.306.