If the school tells the rebbe working in their Yeshiva that the discount for his children is because he’s an employee, then it might be taxable under the new law. If they tell him it is because of his low income they’re discounting the tuition, it is not taxable under the new law just as when they give a low income accountant (working elsewhere) a tuition discount it isn’t taxable.
It’s all a matter of structuring it correctly to legally avoid taxes.