How can a principal who refuses to let students in class fight back because of his personal vendetta?

You can make a complaint to the Education Bureau, because the Education Bureau generally takes over all matters in schools. Be careful to prepare evidence, such as recordings and videos, as well as other illegal evidence, such as in the canteen. Food, now is an era where profit comes first. Generally speaking, the canteen can catch it accurately. The recording and video should highlight the principal's refusal to attend class. Generally speaking, if the evidence is conclusive, the principal will face a high fine. If it is serious, he may be punished. He's not your principal anymore. Don't be merciful when you encounter similar things. Report it. It won't do you any harm anyway. Finally, I wish you success in your studies