Section 77. Sections sixty-nine to seventy-three, inclusive, relating to minors engaged in the occupations mentioned in section sixty-nine, shall be enforced by supervisors of attendance, who are hereby vested with full police powers for the purpose, and by police officers. The school committee of each city may appoint or designate one or more special supervisors of attendance to have supervision over minors engaged in such occupations and over the enforcement of said sections.