Upvote:5
The Immigration Rules V App 3 (25) allow visitors carry out the following:
(a) educational exchanges or visits with a state funded school or academy or independent school; or
(b) a maximum of 30 days study on: (i) recreational courses (not English language training); (ii) a short-course (which includes English language training) at an accredited institution; provided that the main purpose of the visit is not to study and the study is not at a state funded school or academy. https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules
Your school is giving you very bad advice. You need to apply for a student visa.