what subject is this? I'm sure you can redeem yourself depending on how many SACS you have already done and how much your exam contributes to your ss. Don't be too disheartened, there is always an opportunity to improve and you have until the exam for whatever subject you are doing. If you've already identified your mistakes work on fixing them. I know you have said that you probably can't fix it but it would be a bit helpful if you could give some detail.
worst-case scenario, if you REALLY think that you can't get a study score higher than 20, then try and work harder in your other subjects or subjects that you are stronger at so that they can compensate for this subject if you end up with a low ss.
and correct me if I'm wrong, but isn't everyone doing bad an indication of the SAC being too hard? maybe the SAC will scale up? you could also maybe gather a group of people and ask your teacher to scale up all of your scores if the whole class consistently did really bad (like the highest mark being 60 or 50%)
also, remember that ranking matters too!! you can also bring your ranking up and if your cohort is particularly strong, with an exception of this SAC, I think your ss improves or gets a boost or something. (again not sure)