zoo, n.² — Green’s Dictionary of Slang

zoo n.²

[in a scale where A is best, Z is worst]

(US campus) the lowest grade possible; thus as v., to fail an exam.

1979		Eble Campus Sl. Mar. 8: zoo – to fail or perform poorly on a test.
1996		Eble Sl. and Sociability 37: Ace ‘the grade A, Dog ‘the grade D’, and Zoo ‘the lowest grade possible, Z’ are straightforward expansions.