Preserve physical evidence
Keep invoices, receipts and other related receipts, and make sure that they are stamped or signed, so that your evidence will have legal effect.
Multi-person joint proof
If you feel that your personal strength is limited, you can ask several parents to give evidence together. Even if there is no conclusive evidence for the time being, multi-person reaction is also a kind of strength, which can better protect our children and avoid unnecessary involvement.
Report the situation to the newspaper
If you think the problem is serious, you might as well consider reporting the situation to the newspaper to let more people know about it. Let's work together to create a fairer and healthier learning environment for children!