MEMBERSHIP REQUIREMENT.
Tex. GOV'T Code § 822.001.
in effect(a) Membership in the retirement system includes:
(1) all persons who were members of the retirement system on the day before the effective date of this subtitle; and
(2) all employees of the public school system.
(b) Membership in the retirement system is a condition of employment for employees of the public school system unless an employee is excluded from membership under Section 822.002 .
(c) Membership in the retirement system may only be established through employment with a single employer on at least a half-time basis.