Tex. Const. art. VII, § 4

The lands herein set apart to the Permanent School fund, shall be sold under such regulations, at such times, and on such terms as may be prescribed by law; and the Legislature shall not have power to grant any relief to purchasers thereof. The proceeds of such sales must be used to acquire other land for the Permanent School fund as provided by law or the proceeds shall be invested by the comptroller of public accounts, as may be directed by the Board of Education herein provided for, in the bonds of the United States, the State of Texas, or counties in said State, or in such other securities, and under such restrictions as may be prescribed by law; and the State shall be responsible for all investments.