student library assistant; library committee member
A student who serves on the school library committee, helping with book lending, shelf organization, and library events.
I am a library committee member at my junior high school.
The library assistants started lending out the new books.
図書室 is the library room itself, while 図書委員 is the student who works there as a committee member.
司書 is a professional librarian, whereas 図書委員 is a student volunteer or committee member.
Compound of 図書 (library books) and 委員 (committee member). A straightforward modern school term.