The minimum requirements for safety for school buildings in existence prior to March 18, 1924 shall consist of easy means of exit from not less than two opposite sides of the school building and when in case of an old building there is only one stairway there must be provided, before any such school can be legally operated, another stairway for exit on the opposite side of the building or a safe slide securely fastened to the building and extending from one or more windows on the side opposite the stairway and reaching to within six feet of the ground. All hall and exit doors shall open outward. No public moneys may be legally paid out for the operation or maintenance of any existing public school where there are hanging flues in the building or where the stovepipe runs through the side of the building, a window or the roof, unless the same be safely encased in a brick flue or double terra cotta.