(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.