34 Tex. Admin. Code § 25.81
A member may obtain out-of-state service credit for qualified employment in public educational institutions which are maintained in whole or in part by one of the states in the United States of America; by a commonwealth, territory, or possession of the United States of America; or by the United States government. Public educational institutions of the United States government must have been maintained for the primary purpose of educating the children of United States citizens either in foreign countries or in locations within the United States where state and local government have not provided public educational facilities. The service in eligible institutions must satisfy the requirements for membership in the Teacher Retirement System of Texas, except for the requirement that the employment be in Texas. Further, the service must have been for at least 4 1/2 months of the school year, or for at least a full semester of more than four calendar months, or for at least 90 days of a school year as a substitute in a position otherwise eligible for out-of-state service. For service rendered in the 2011-2012 school year and after, a member must have worked or received paid leave in a position otherwise eligible for out-of-state service credit for at least the minimum amount of time 90 days required of a member to receive service credit in that school year or worked for at least 90 days in a school year as a substitute in a position otherwise eligible for out-of-state service credit. A member may satisfy any of these requirements by combining the out-of-state service with employment in the Texas public schools that occurred in the same school year and for which deposits are maintained in the member's account. A member eligible to establish normal membership service credit for a school year may not obtain out-of-state service credit for that year.
34 Tex. Admin. Code § 25.81