All minors under the age of 16 years must attend an all-day school, as that term is used in this title, or be legally employed. Every parent, guardian or other person having custody and control of a child between the ages of 12 and 16, who has been granted a certificate to leave school to engage in a legal employment shall cause such a child to attend a part-time school or class for a period the equivalent of at least 4 hours each week for at least 36 weeks in each year while so employed. When such minor is temporarily unemployed, the hours of attendance upon such part-time school or class shall be at least 20 hours per week.