The Commissioner, to the extent necessary in order to prevent curtailment of opportunities for employment, shall by regulations provide for:

(a) the employment of learners, of apprentices, and of messengers employed primarily in delivering letters and messages, under special certificates issued pursuant to regulations of the Commissioner, at such wages lower than the minimum wage applicable and subject to such limitations as to time, number, proportion, and length of service as the Commissioner shall prescribe; and

(b) the employment of individuals whose earning capacity is impaired by age or physical or mental deficiency or injury, under special certificates issued by the Commissioner, at such wages lower than the minimum wage and for such period as shall be fixed in such certificates; and

(c) any individual employed by any state, county, city, town, municipal corporation or quasi-municipal corporation, political subdivision, or any instrumentality thereof; and

(d) students and regular attendants at any institution of higher learning, either public or private.