Although from a strict, computer-like grammar perspective you'd be right; stylistically it is more effective to use an apostrophe in this case in order to avoid confusion with the word "As." I suspect if you asked a hundred random people about "Straight As" a goodly number would feel weird reading it, or would actually mistake the meaning. Conversely, "Straight A's" is only going to bother people who have memorized the rules.
Drop that apostrophe!