Section 102. If a minor shall, without the orders, consent or knowledge of the employer or of the superintendent, overseer or other agent of the employer, labor in a manufacturing or mechanical establishment, factory or workshop during a part of any time allowed for meals in such establishment, factory or workshop, according to the notice required by section fifty-six, and if a copy of such notice was posted in a conspicuous place in the room where such labor was performed, with a rule of the establishment, factory or workshop forbidding such minor to labor during such time, neither the employer nor a superintendent, overseer or other agent of the employer shall be held responsible for such labor.