(a) In this subchapter, "professional employee of a school district" includes:
(1) a superintendent, principal, teacher, including a substitute teacher, supervisor, social worker, school counselor, nurse, and teacher's aide employed by a school district;
(2) a teacher employed by a company that contracts with a school district to provide the teacher's services to the district;
(3) a student in an education preparation program participating in a field experience or internship;
(4) a school bus driver certified in accordance with standards and qualifications adopted by the Department of Public Safety of the State of Texas;
(5) a member of the board of trustees of an independent school district; and
(6) any other person employed by a school district whose employment requires certification and the exercise of discretion.
(b) The statutory immunity provided by this subchapter is in addition to and does not preempt the common law doctrine of official and governmental immunity.