Tex. Education Code § 55.21: BONDS ARE AUTHORIZED INVESTMENTS AND SECURITY FOR DEPOSITS.

All bonds issued by any board are legal and authorized investments for all banks, trust companies, building and loan associations, savings and loan associations, insurance companies of all kinds and types, fiduciaries, trustees, and guardians, and for all interest and sinking funds and other public funds of the State of Texas, and for all agencies, subdivisions, and instrumentalities thereof, including all counties, cities, towns, villages, school districts, and for all other kinds and types of districts, public agencies, and bodies politic. Said bonds also shall be eligible and lawful security for all deposits of public funds of the State of Texas and all agencies, subdivisions, and instrumentalities thereof, including all counties, cities, towns, villages, school districts, and all other kinds and types of districts, public agencies, and bodies politic, to the extent of the market value of said bonds, when accompanied by any unmatured interest coupons appurtenant thereto.

Acts 1971, 62nd Leg., p. 3072, ch. 1024, art. 1, Sec. 1, eff. Sept. 1, 1971.