Upvote:2
It depends on the course of study you're pursuing. If it is less than six months, or if it is an English language course and less than 11 months, you can apply for a student visitor visa:
https://www.gov.uk/study-visit-visa
If the course of study is longer, you should apply for a (Child) student visa:
https://www.gov.uk/child-study-visa
The pages don't say it explicitly, but both of these visas must permit multiple entries.