Message saying "School is up!" (4)

I believe the answer is:

note

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'message saying' is the definition.
'note' can be an answer for 'saying' (thesaurus). I'm unsure of the 'message' bit.

'school is up' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'is up' says the letters should be written backwards (in down clue: letters go upwards).
'eton' in reverse letter order is 'NOTE'.

Can you help me to learn more?

(Other definitions for note that I've seen before include "Reminder - fame - sound" , "Pay close attention to" , "Tone of a short record" , "Cash" , "Observe in Eton" .)

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

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