It's obviously not good to refuse in person. After all, he is a teacher. If he wants to raise this matter to the level of parents, you should not be able to refuse. So you need to communicate with your parents first to express your unwillingness to go and get support. Then on Saturday, your parents will call your teacher and say that you can't go because of something. As long as your teacher has a little worldly wisdom, he must have figured out the difference between the two.