Nothing contained in this article shall abrogate the power to set up rental or free textbook systems of any school district or county that may have such right, and all rentals received by any such district or county from rental of books belonging to such district or county shall be retained by such district or county. And school districts enrolling five thousand or more school pupils shall have the right to set up rental or free textbook systems and choose and purchase their own textbooks. All rentals received by such districts from rental of books belonging to such districts shall be retained by such districts. But any such county or school district having its own rental or free textbook system shall be entitled to receive all benefits under the rental library provisions of this article.