Why couldn't they find out already what spell the Professor had in mind for that class. Oh, another question, she thought and tried to find an answer as quick as possible.
'We can use Finite Incantatem', the Ravenclaw said after having raised her hand. The counter curse might be longer... or one might forget it. It's impossible to know ALL spells, and all counter curses. Psh. Finite incantatem should work on anything.