No person under the age of eighteen years shall be employed by any messenger company, in cities having a population of twenty thousand or over, to distribute, transmit or deliver goods or messages between the hours of ten o’clock at night and five o’clock in the morning. The manager of the office of any corporation who violates any provision of this section shall be fined not more than fifty dollars for each day of such employment. The provisions of this section shall not apply to persons under the age of eighteen who have graduated from a secondary educational institution.