(b) The secretary, to the extent necessary to prevent curtailment of employment opportunities, shall by regulations provide for the employment of learners and students, under special certificates at wages lower than the minimum wage applicable under this section, and subject to such limitations as to number, proportion and length of service as the secretary shall prescribe: Provided, That the minimum wage prescribed under this subsection (b) shall not be less than eighty-five percent of the otherwise applicable wage rate in effect under section 4. A special certificate issued under this subsection shall provide that for six or less students for whom it is issued shall, except during vacation periods, be employed on a part-time basis and not in excess of twenty hours in any workweek at a sub-minimum rate.
In the case of an employer who intends to employ seven or more students, at a sub-minimum rate, the secretary may issue a special certificate only if the employer certifies to the secretary that employment of such students will not create a substantial probability of reducing the full-time employment opportunities for other workers.