Longman Dictionary of Contemporary English
head boy
noun
EXAMPLES FROM CORPUS
▪ After all, the head boy must show the way.
▪ As head boy, you know.
▪ He was educated at the City of London School, leaving at the age of sixteen as head boy.
▪ Hilary Frome was, after all, head boy designate.
▪ One of its main decisions was to abolish the school's head boy and girl system.
Wiktionary
head boy
n. (context British English) A boy of a school sixth form, whose duties may include representing the school and organising the prefects.