Section 40. Whoever wilfully interrupts or disturbs a school or other assembly of people met for a lawful purpose shall be punished by imprisonment for not more than one month or by a fine of not more than fifty dollars; provided, however, that whoever, within one year after being twice convicted of a violation of this section, again violates the provisions of this section shall be punished by imprisonment for one month, and the sentence imposing such imprisonment shall not be suspended.