Head of school; a goody-goody youth (5)
I believe the answer is:
sprig
'youth' is the definition.
(I've seen this in another clue)
'head of school a goody-goody' is the wordplay.
'head of' says to take the initial letters.
'a goody-goody' becomes 'prig' (both can mean a sanctimonious person).
The initial letter of 'school' is 's'.
's'+'prig'='SPRIG'
(Other definitions for sprig that I've seen before include "Small stem bearing flowers" , "Small stem taken from a bush or plant" , "related to Spike and Brad" , "Spray or twig" , "Small branch or shoot" .)