The expenses of maintaining public schools in each town, which shall be incurred with the approval of the board of education, shall be paid by the town treasurer on orders drawn by said board, except so far as they may be met by the income from local school funds. Such orders may be signed by such persons on behalf of the board as the board by bylaw or special vote, certified by the secretary to the town treasurer, provides; and, in the absence of such bylaw or special direction, by the secretary.