First of all, don't worry about the C2 Education score. After I already got a 2000+ on the real SAT test (took as a trial in middle school), I went there at the end of 9th grade and the people said I only got a 1800. -_____- (btw, I'm also a rising junior)
I didn't really trust them, so I decided to study on my own. I didn't really do that much prep over the years, but I just gradually built up my score by looking at vocab occasionally, doing a practice test every 1-3 months, etc. Also, I found that going online to read random news articles helped my reading score immensely. Even reading those trashy popular teen novels will help build your reading base, especially if you jot down some vocab you aren't too sure about as you read.
I took the real SAT a second time in January and made a 2300+. To be honest, before January, I hadn't taken a practice test in about half a year. For the month up to the test, I just flipped through the Barron's SAT book (which IMO is the BEST book out there) for quick grammar tips and for vocabulary (about 5000 words in there!). I also took vocab online quizzes and such. Practice IS important, but not so much as your "base" (how good your reading skill is, how well you can write, did you memorize vocab?, etc.)
I'm planning to take the October SAT test for that 2400, so I guess I'm trying to actually get into that "testing mode" now. For now, I'm just reading whatever I can get my hands on and memorizing vocab. (:
For writing, I'm just reviewing grammar rules and occasionally writing an essay.
I actually don't have much to say about preparing for the math section, but just make sure you know your algebra well. Actually, since you can bring a calculator into testing, you shouldn't worry too much about it.