I did the bare minimum amount of homework (i.e. enough to not get a detention) and absolutely no study in years 7-10 - Apart from coming equal first in music one year my grades were average at best. Over the summer holidays between year 10 and 11 I developed a passion for physics and realised that if I wanted to study it at uni I really needed to pull my shit together. So I did.
I started doing homework and revision and soon enough I was doing much, much better than before. After the half yearlies I was 5th in 3 unit maths, 19th in 2 unit, 4th in legal and 12th in Physics. It really didn't seem like I was doing a lot more work, but a little bit of study makes a lot of difference - especially when you're in the position I was in/you're in now. Then you gradually apply yourself more throughout the year and hopefully catch up to everyone else and go straight past them - I'm topping 3 subjects at the moment.
It's never too late to start applying yourself to school, although the earlier you do it, the better. Most importantly, there's no point changing if you're not going to enjoy it.
In regards to advice for specific subjects, this is what I've found worked for me:
Maths - Keep doing problems until you understand the concept. This was really hard at the start of year 11 (I'd paid no attention in year 10 whatsoever) because I didn't know much more than basic algebra and even then I wasn't confident in my ability. Get the basics down first (expanding/factorising, then move on to basic trig, calc etc.) and then move on to the more "advanced" topics - You're not going to be able to do locus & the parabola if you can't simplify algebraic expressions. Again WORK UNTIL YOU HAVE AN IN-DEPTH UNDERSTANDING OF THE TOPIC, whether it takes you 10 minutes or 10 days. As to how to get that understanding, I'm not sure, past papers/textbook exercises work for me but not for my friends. I'm sure Spiral can help you out there.
Physics & Chemistry - Personally I think you should be gaining an in-depth understanding of the entire course and even going further than what's required (for example, looking at the derivations of the formulas you learn rather than just accepting them as fact) but the way it's structured you could easily just rote learn the entire textbook and end up with a band 6 - I know that's what half my chem class did. (This would work for Bio as well probably; from what I understand the science courses are fairly similar although I have no experience with bio)
As for English I'm not the best source of information, but I'll try to help anyway. Memorise paragraphs rather than whole essays. By this I mean have a list of your quotes/techniques/effects and work out how you would fit them into an essay. You still go into the exam knowing what to write, but you're not screwed if the question doesn't fit what you've memorised.
tl;dr just apply yourself, you'd be surprised how easy it is once you get started.
