Their justification? The word "dinosaur" calls to mind other no-no words, like "evolution" (also on the proposed list of forbidden words), which could offend test-takers who might not necessarily believe in evolution. Or... dinosaurs?
The fear, according to this report from CBS New York, is that certain words and topics could make students "feel unpleasant" while they're taking city-issued tests. Other words on the prospective ban-list include: "Halloween" (suggests paganism), "birthday" (insensitive to Jehovah's Witnesses, who don't celebrate birthdays), "dancing" (but not "ballet," which the city has evidently made an exception for), "video games" and "rap music."