Classes very restless, teacher stressed at first (5)
I believe the answer is:
sorts
'classes' is the definition.
('sort' can be a synonym of 'class')
'very restless teacher stressed at first' is the wordplay.
'very' becomes 'so' (I've seen this in other clues**).
'at first' indicates taking the first letters.
The initial letters of 'restless teacher stressed' is 'rts'.
'so'+'rts'='SORTS'
(Other definitions for sorts that I've seen before include "Varieties" , "Arranges in correct or desired sequence" , "Kinds" , "Segregates items into groups" , "Arranges in different groups" .)