Away from classes, slightly unwell (3,2,5)
I believe the answer is:
out of sorts
'slightly unwell' is the definition.
(I've seen this before)
'away from classes' is the wordplay.
'away from' becomes 'outof'.
'classes' becomes 'sorts' ('sort' can be a synonym of 'class').
'outof'+'sorts'='OUT OF SORTS'
(Other definitions for out of sorts that I've seen before include "Slightly unhappy" , "Not feeling good" , "Lacking variety" , "Slightly unwell" , "under the weather" .)