Middle school destined for firstborn (6)

I believe the answer is:

oldest

I'm a little stuck... Click here to teach me more about this clue!

'for firstborn' is the definition.
(I've seen this in another clue)

'middle school destined' is the wordplay.
'OLDEST' is hidden within the letters.
However, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for oldest that I've seen before include "Eldest" , "Most senior in age" , "Most aged" , "Least young" , "First in time" .)

I've seen this clue in the Sydney Morning Herald.

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