Relating to one in school that’s unfriendly? (6)

I believe the answer is:

offish

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'unfriendly?' is the definition.
(offish can mean aloof or unfriendly)

'relating to one in school' is the wordplay.
'relating to' becomes 'of'.
'one in school' becomes 'fish' (I can't explain this - if you can you should believe this answer much more).
'of'+'fish'='OFFISH'

'that's' acts as a link.

Can you help me to learn more?

(Another definition for offish that I've seen is " Aloof, distant in manner".)

I've seen this clue in The Guardian.

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