I left school ultimately in poor health (3)
I believe the answer is:
ill
'in poor health' is the definition.
(I've seen this in other clues)
'i left school ultimately' is the wordplay.
'left' becomes 'l' (common abbreviation).
'ultimately' says to take the final letters.
The final letter of 'school' is 'l'.
'i'+'l'+'l'='ILL'
(Other definitions for ill that I've seen before include "Evil or sick" , "Off-colour" , "Not well, sick" , "Seasick" , "Faulty" .)