Female, one in school, is looking humourless (2-5)
I believe the answer is:
po-faced
'looking humourless' is the definition.
(po-faced means overly prudish or disapproving)
'female one in school' is the wordplay.
'female' becomes 'f' (abbreviation).
'one' becomes 'ace' (ace has a value of one in some card games).
'in' means one lot of letters goes inside another.
'school' becomes 'pod' (I've seen this in other clues).
'f'+'ace'='face'
'face' inserted within 'pod' is 'PO-FACED'.
'is' is the link.
(Other definitions for po-faced that I've seen before include "Not showing amusement, having solemn expression" , "Looking unamused" , "Humourless and disapproving" , "With disapproving look" , "Wearing a stern expression" .)