Tex. Educ. Code § 66.25

The carrying out of the purposes of the constitutional provisions and of this subchapter will be performing an essential public function under the constitution, and all bonds and notes, whether original or refunding, heretofore or hereafter issued pursuant to the constitutional provisions or this subchapter, and their transfer and the income from them, including the profits made on their sale, shall at all times be free from taxation of this state.