Excellent start at school? (5-5)

I believe the answer is:

first-class

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

'excellent' is the definition.
(I know that excellent can be written as first-class)

'start at school?' is the wordplay.
'start' becomes 'first' (synonyms).
'at school?' becomes 'class' (I can't justify this - if you can you should believe this answer much more).
'first'+'class'='FIRST-CLASS'

Can you help me to learn more?

(Another definition for first-class that I've seen is " Top flight".)

I've seen this clue in The Globe and Mail.

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