1. "Bringing bills" is an illegal act, which violates the principle of "fairness and good faith" in the Law of People's Republic of China (PRC) on Anti-Unfair Competition. Therefore, it is illegal for teachers to buy big and small.
2. Bringing a bill is not for buying, using goods or receiving services for daily consumption, so even if the teacher brings a bill to buy big and buy small, it is not protected by the Consumer Protection Law.