34 Tex. Admin. Code § 25.4
Employees who substitute in a position otherwise eligible for membership may qualify for membership provided that they are employed for at least 90 days in the school year. Such service shall be considered the equivalent of at least four and 1/2 months of service. Members claiming credit for such service will be assessed a fee for delinquent deposits, if applicable, as provided in §25. 43 of this title (relating to Delinquent Deposits).
Source Note:The provisions of this §25.4 adopted to be effective January 1, 1976; amended to be effective January 1, 1979, 3 TexReg 4439.