If he signed you up without your consent, then don't bother doing it. If he complains, point out that, technically, he should've ASKED first, and done so with at least a WEEK'S advanced notice. You aren't there to do things outside the class with ZERO notice and VERY LITTLE preparation.
EDIT: I realize now that it's a bit late for what I said, but it's still incredibly bad form for him to spring it on you like that. Let your guidance councilor know that, too.