Important:
This answer is based on tax law year ending 28 February 2017.
Answer:
This is not really a tax related questions and you may well want to refer the question to the National Treasury.
According to section 3 of the Employment Tax Incentive Act an employer is eligible to receive the employment tax incentive if the employer:
(a) is registered for the purposes of withholding and payment of employees’ tax by virtue of paragraph 15 of the Fourth Schedule to the Income Tax Act; and
(b) is not—
(i) …
(ii) a public entity that is listed in Schedule 2 or 3 to the Public Finance Management Act, 1999 (Act No. 1 of 1999), other than those public entities that the Minister of Finance may designate by notice in the Gazette on such conditions as the Minister of Finance may prescribe by regulation;
(iii) …
So the requirement is not school is a private school or not, but rather whether the employer is a public entity. From the facts it appears that the persons are not employed by the school.