Wow that is quite rough, my year level is the same, class averaged 24% on our first SAC, which was like basic stuff from last year...
If you are looking to improve your math, then honestly u r a step ahead than a lot of kids. First thing I did was figure out how I studied math, and there are 2 ways. 1) If you are a formula guy, then look at how the textbook solves a variety of problems in the chapter, get it to a dot point form or list and use that. If u do that, then notes, whilst important, come after you have done the questions and figured out how to answer each style. Next style is concepts, instead of just knowing the general quadratic form, really try to understand it. Go onto Desmos graphing and play around with variables, just try to understand something like when you are solving a linear equation, you may often have real world context and so be realistic and think logically. If you are more of a concept guy, I reccomend writing notes first and then doing the excersizes once u understand the theroy. After this, it is all up to practice, practice, practice. It sucks, but I did every question from a few important exercises and filled up a 128 book (I have very large/messy writing tho) just for 3.5 chapter SAC, which I got 95% on, much higher than the average, but like don't stress too much, just do enough so that they seem repetitive and u r confident. Also ask questions!!!!!!! Teacher knows, teacher writes SAC, or maybe ask some people on here.
TLDR: Figure out ur style of study, do questions but don't drill urself to death and ask questions!