It depends on your writing size, but that is probably too little. Ideally, you should be writing about 1000 words, or 7+ pages with average hand writing.
You are going to do EXACTLY the same subjects as I am currently doing for my HSC. However, in year 11 I also did music (14 units was wayy too much).
You'll be fine with your choices.
Terrible to admit, but true.
I love my school and my teachers are amazing, but the range of students that you get are...diverse - to say the least. =_="
Unless you are absolutely amazing (by which I mean, out of this world), you really do need to write more than 3.5 pages to make a coherent and credible argument with your essay.
I was thinking that, but they would have to write pretty darn small (maybe too small for a marker?). Mine is pretty small-medium and I normally do about 7.5-8 with about 1200...