Stylish pupils - unknown (6)
I believe the answer is:
classy
'stylish' is the definition.
(similar in meaning)
'pupils unknown' is the wordplay.
'pupils' becomes 'class' (I've seen this before).
'unknown' becomes 'y' (algebra).
'class'+'y'='CLASSY'
(Other definitions for classy that I've seen before include "Stylish (slang)" , "Sophisticated, stylish" , "showing style ..." , "Sophisticated (colloq.)" , "Elegant and fashionable, posh" .)