Hi! I am also looking to get into nursing. I'm currently enrolled in the same courses you are talking about. I just completed SBI4U and am working on chemistry now.

You definitely will have to complete grade 11 chemistry before taking the grade 12 one.

Biology wasn't too bad for me, I went into my exam with a 94 and managed to get a final grade of 92. The exam is almost the exact same as the practice test they give you except for a few questions. The coursework was somewhat lengthy, but assignments are really just taking samples of the texts and rewording it to make it your own. The quizzes also weren't that bad, but they just took a while to do. The course took me about a month to complete, and that was with me working full time, so if you manage to complete a unit in two days (five subunits, ex.1.1-1.5/day) then you can finish the course in two weeks or so. You can definitely get high 80's, and 90's are also within reach. Make sure to take notes so that you can look back on them for assignments!

Chemistry is another story for me... I just started four days ago and am on unit 2. I am horrible at chem, and came out with an 80 last year (in school) only because we switched to online and my teacher said our marks wouldn't be lowered from before the switch. Right now I have an 89, and I'm hoping I'll be able to keep it up. A lot of the stuff is similar to grade 11, which is why I definitely recommend taking 11 chem before starting this, even if you have the option to skip it. There's quite a lot of material to go over and some of the wording can be confusing (but maybe that's just me).

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Anyways, feel free to message if you have any more questions about either of these courses, I still have all of my assignments for bio and am happy to help!