Master trained pupils of similar ability (6)
I believe the answer is:
stream
'pupils of similar ability' is the definition.
(stream can mean a class of similarly able pupils)
'master trained' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'master' anagrammed gives 'STREAM'.
(Other definitions for stream that I've seen before include "Rill, brook" , "Natural body of running water" , "Flowing water" , "not taken in mixed-ability group" , "Constant flow of liquid" .)