Section 9A. Upon application of twenty or more residents of any city or town who are eighteen years of age or over certifying that they desire to attend a class for instruction of the type described in section nine, the school committee of such city or town shall establish and cause to be conducted such a class or classes for a period of not less than forty sessions; provided, that, in case the attendance of any such class falls below fifteen, such school committee may discontinue such class.