Fish out a key to the school, girl! (2-2)
I believe the answer is:
Here is my best explanation:
'the school girl' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'fish out a key' is the subsidiary indication.
'fish' becomes 'cod' (cod is a kind of fish).
'out' indicates putting letters inside.
'a key' becomes 'e' (I've seen this in other clues).
'cod' placed around 'e' is 'coed'.
'to' is the link.
(Other definitions for co-ed that I've seen before include "Teaching attended by both sexes", "Teaching both girls and boys", "School for both sexes" and "School for boys and girls, in short".)