In our school pretty much every student is interested in being a librarian, so I had to introduce a selection process. Have a look at the 'student librarian' page on this blog where I have added my application form and some information about the process.
Our group of student librarians is chopped up into 2 different kinds: class librarians who are 9 and 10 years old (they do the issuing, returning and shelving of books for their class when they are having their weekly library time) and lunchtime librarians who are 7 and 8 years old (they look after the library desk where puzzles and board games are issued during lunchtime).