Could be excessively so for school, weatherwise or attitude-wise. (4)
I believe the answer is:
cool
'could' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for cool that I've seen before include "chic" , "Hip - not warm" , "Chilled" , "Lovely" , "Quite cold" .)