Our teacher doesn't use the textbook at all and tells us to look at the textbook in our own time. She's not a textbook teacher, she instead makes up questions for us to do, uses Checkpoints and gives us practice exam papers from, like, Insight and stuff like that. The SAC wasn't hard, but it wasn't easy, either and we all ended up doing pretty well... the lowest mark was a B, I think.
It SHOULDN'T have much of an impact on your study scores if you do well in the exam -- it could just be that your teacher sets insanely hard SACs... some do that. My methods teacher did that last year. To be honest, though, SACs are just a ranking, and it's only the first one, so I wouldn't worry too much if you haven't done well in it.