34 Tex. Admin. Code § 25.41
Persons who have been required by law to be members of the Teacher Retirement System of Texas (TRS) or who have service or compensation on which contributions were required but who have not made the required deposits shall start making deposits immediately for current service and in addition shall make required contributions on any service rendered or compensation received earlier in the current school year. If the service was rendered and/or compensation was received in a prior school year, except as provided in § 25.28(g) of this title (relating to Payroll Report Dates), the member may make payment of the actuarial cost to establish the previous service or compensation under § 25.43 of this title (relating to Cost for Unreported Service or Compensation).
34 Tex. Admin. Code § 25.41