I'm Peruvian, 15 years old, and travelling to the UK to visit school.
I will be arriving in the UK, then departing to the Vatican for a week, then back to the UK again to resume my school visit. What visa should I apply for?
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:
If the course of study is longer, you should apply for a (Child) student visa:
The pages don't say it explicitly, but both of these visas must permit multiple entries.