I was in the same boat as you at the start of the year - seemed no matter how hard I tried I couldn't get over a particular score. Was also slightly disappointed in my midyears result. However, I've managed 25/25 on my last two SACs because now I get what they are actually looking for. Sometimes, especially in bio, answering the question is not enough. You need to show off your knowledge, so with every answer I've started putting every little detail relating to the question in and it seems to be working. Hopefully it'll work for the end of year exams as well. I know they say they want concise explanations, but sometimes if it isn't clear what the question is after then too much information is better than not enough.
Anyway, not all is lost! You can still improve your scores and once you get what your particular teacher is after then SACs shouldn't be a worry. If they give your SACs back, make sure you take note of everything you did wrong so you're not making the same mistake twice.