Section 7L. Notwithstanding the provisions of clause (2) of section seven B, in any city or town which accepts the provisions of this section, the operator of a school bus shall not allow the number of school children riding in the bus at any one time to exceed the number of seats therein, nor shall he drive said bus until each child is seated. Whoever violates the provisions of this section shall be punished by a fine of not less than fifty dollars nor more than five hundred dollars.