Section 40A. Whoever gives, sells, delivers or has in his possession any alcoholic beverage, except for medicinal purposes, in any public school building, or on any premises used for public school purposes and under the charge of a school committee or other public board or officer, shall be punished by imprisonment for not more than thirty days or by a fine of not more than one hundred dollars, or both; provided, however, that a school committee of a city, town or district may authorize a public or nonprofit organization using a public school building with its permission during non school hours to possess and sell alcoholic beverages therein provided such nonprofit organization is properly licensed under the provisions of section fourteen of chapter one hundred and thirty-eight.