Green’s Dictionary of Slang

(Aus. prison) the Classification Committee; the classification a prisoner receives.

1988		‘Siriusly’ Speaking in Just Us (H.M. Prison, Beechworth, Vic.) Dec. 25/2: classo 1. n. Travel agency (I’m going to see classo tomorrow). 2. v. To be classified as suitable for, say, a particular prison (the bastards have classoed me to Beechworth).
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Classo. 1. The Classification Committee. This committee assesses prisoners and makes recommendations on their placement in the system. 2. As in ‘A, B, or C classo’, ie the security classification assigned a prisoner.
2005		M.B. ‘Chopper’ Read Chopper 4 168: Imagine me marrying into the classo board.