Does it violate the labor law to deduct personal income tax from the part-time salary of Heze KFC student workers?

It has nothing to do with labor law. According to the new tax law, part-time income is taxable, but what you said seems to be that there is no other income and it can't reach 3000 yuan, so it should not be deducted. Please note that student internship is not protected by contract law, and student work is not a formal term. It is illegal to make a full-time salary lower than that of 950 yuan (minimum wage) or an hourly salary lower than 10 yuan.