Tex. Education Code § 52.13

All bonds issued pursuant to the provisions of this chapter are legal and authorized investments for banks, savings banks, trust companies, building and loan associations, insurance companies, fiduciaries, trustees, and guardians, and for the sinking funds of cities, towns, villages, counties, school districts, and all other political subdivisions and public agencies of the State of Texas. The bonds, when accompanied by all unmatured coupons appurtenant to them, are lawful and sufficient security for all deposits of state funds and of all funds of any agency or political subdivision of the state, and of counties, school districts, cities, and all other municipal corporations or subdivisions at the par value of the bonds. The bonds and the income from them, including the profits made on their sale, shall at all times be free from taxation in this state.