Clue: Idle talk outside of tower with old school class member (9)
Answer: GASTROPOD
Explanation: Definition: class member
 GAS(idle talk/hot air) + **1st and last letters of**(outside of) "tower" **plus**(with) O(abbrev. for "old") + POD(collective noun for a herd/school of marine animals, say, whales).
Defn: A member of the class of invertebrate organisms, the Gastropoda, that includes snails, slugs and marine shellfish.