Go directly to the school to apply for withdrawal. The school will charge your tuition according to the length of your school time. If you overpay, they will return it to your bank card. The school will contact you by email.

Visa cancellation is handled by the school, not you. When you choose to quit school, you will sign a withdrawal agreement. Sign it and quit school. Then the relevant department of the school will send a message to the visa office that you have dropped out of school, and then the visa office will send you an email telling you to leave the UK within a certain period of time, usually within one month, in order to prevent you from working here or staying here.

Is there no international support team in your school? You ask, this should be available in most schools. You talk to them, and they will give you a more detailed answer.