Longman Dictionary of Contemporary English
head girl
noun
EXAMPLES FROM CORPUS
▪ By all accounts Selina was a bright child who ended up as head girl of her secondary school in Guisborough near Middlesbrough.
▪ I was made head girl there and captain of the senior hockey team.
Wiktionary
head girl
n. (context British English) A girl of a school sixth form, whose duties may include representing the school and organising the prefects.