As of August 2009, the Texas Education Agency released this statement about whether kindergarten was mandatory in the state:
No. Children who are five years of age on or before September 1 are not required to enroll in kindergarten. However, on enrollment in kindergarten, a child shall attend school. Kindergarten students are subject to compulsory school attendance rules while they are enrolled in school. If a child has not reached mandatory compulsory attendance age (6 years old as of September 1 of the current school year) the child’s parent or guardian may withdraw the student from school, and the child will not be in violation of compulsory attendance rules.
I couldn't find information to the contrary online, so I'm fairly confident this policy is still in place.
Download the Kindergarten FAQ's PDF at the link listed in the Resources section below for more information about kindergarten guidelines for the state of Texas.