Hardly a finishing school! (4)

I believe the answer is:

prep

I'm a little stuck... Click here to teach me more about this clue!

'school' is the definition.
(prep school is a kind of school)

I cannot understand how the remainder of the clue works.

(Other definitions for prep that I've seen before include "Homework (abbrev.)" , "Homework (abbr.)" , "warm-up race" , "School work done outside lessons" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app