The board of education in every school district in this State in which there are employed or there reside 15 or more children between the ages of 12 and 16 years to whom have been granted employment certificates in accordance with the child labor and compulsory school attendance laws shall establish and maintain part-time schools and classes in general civic or vocational subjects for such employed children or for minors under 16 who have ceased to attend all-day schools. Such schools and classes shall be under the control and management of the board of education and shall be an integral part of the public school system of the city or district which maintains them. The part-time schools or classes shall be maintained each year during the full period of time when the public schools of the district are in session.