Cassie opens her mouth to somehow convince the professor that they weren't doing or discussing any sort of mutiny but then she feels the dampening charm and closes her mouth.

"Good evening, Professor," she says watching her then looks at Draca before returning her attention to McGonagall.

How much did she hear? How much could she say? "The ball is fine. It's the tournament itself that leaves much to be desired..." And the rest of the wizarding world as it stands.