19 Tex. Admin. Code § 230.105
A teacher who holds a valid provisional, professional, standard, or enhanced standard classroom teaching certificate or a valid temporary classroom teaching certificate issued under the provisions of Subchapter H of this chapter (relating to Texas Educator Certificates Based on Certification and College Credentials from Other States or Territories of the United States), or Chapter 245 of this title (relating to Certification of Educators from Other Countries), and a bachelor's degree or higher from an accredited institution of higher education may qualify for an additional teaching field or certification to teach at another level by passing the appropriate certification examination(s) for that subject. The teacher must submit the application to add certification based on an examination during the time the certificate is allowed to be issued by the State Board for Educator Certification. The application for the additional certification must be submitted during the validity period of the appropriate Texas classroom teaching certificate. If a teacher holds multiple teaching certificates, all teaching certificates must be active before adding certification by examination. The rule shall not be used to qualify a classroom teacher for:
19 Tex. Admin. Code § 230.105