The boards of supervisors of the several counties of this state may, in their discretion, pay such part of the cost of labor and material for laying water mains from municipal water systems along the highways outside the corporate limits of such municipalities, not exceeding a distance of five miles, as in their discretion, may be to the public interest of their respective counties. Such expenditure on the part of a board of supervisors shall not exceed one-half of the entire total cost of labor and material for laying such water main.
However, no part of said expense is to be borne by any board of supervisors unless there is situate on the route of such proposed water main one or more county schools.