The Secretary of the Interior is hereby authorized, upon application by the proper officers of a school district located wholly or in part within the boundaries of a project of the United States Reclamation Service, to issue patent conveying to such district such unappropriated undisposed of lands, not exceeding six acres in area, within any Government reclamation town site situated within such school district as, in the opinion of the Secretary of the Interior, are necessary for use by said district for school buildings and grounds: Provided, That if any land so conveyed cease entirely to be used for school purposes title thereto shall revert to and revest in the United States.