Short boy skipped half of class (3)
I believe the answer is:
les
'short boy' is the definition.
(short for Leslie; 'short' suggests a short form)
'skipped half of class' is the wordplay.
'skipped half of' suggests halving the letters.
'class' becomes 'lesson' (I've seen this before).
'lesson' halved is 'LES'.
(Other definitions for les that I've seen before include "- Dawson, Br. comedian" , "Short boy or girl" , "-- Dawson, English comic" , "Boy's name (abbr)" , "-- Dawson (late comedian)" .)