All lands heretofore, or hereafter granted to the several counties of this State for educational purposes, are of right the property of said counties respectively, to which they were granted, and title thereto is vested in said counties, and no adverse possession or limitation shall ever be available against the title of any county. Each county may sell or dispose of its lands in whole or in part, in manner to be provided by the Commissioners Court of the county. Said lands, and the proceeds thereof, when sold, shall be held by said counties alone as a trust for the benefit of public schools therein; said proceeds to be invested in 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 counties shall be responsible for all investments; the interest thereon, and other revenue, except the principal shall be available fund.