A lot of people above has pretty much said everything that needs to be done to 'study smarter' but i'll summarise my opinion of studying smarter.
- Understand what you're studying: There's no real point in skimming through your content every time you're studying and saying 'i know this - next' because you're wasting time and you're giving yourself false confidence. If you really want to test yourself i'd recommend looking for specific questions that really test you, that makes you think - i know this but to what extent?
- Ask for help - honestly, you're at school to learn - you're not expected to know everything. If you did know everything, you could just skip all the classes and take tests. Students often refuse to ask for help because they think their peers or teachers would think they're stupid or for whatever reason. Worst thing i did - look at the answers and tried to logically explain it to myself. Yes, it did help me understand concepts BUT my understanding wasn't the most clearest understanding because i just worked backwards. Big mistake. I lost like 1-2 marks in the 7 markers because a lot of my answers weren't succinct as they could be.
- Reflect upon your mistakes - when you get a difficult question incorrect: analyze the fuck out of it. Look at the dotpoints that it used, the concepts, etc. Often harder questions deepen your understanding and just reading the answer IMO doesn't help as much.
- Take breaks: HSC in retrospect didnt require that much work. I could've easily maintained a part/ full time job if i just managed my time since majority of it was just the stress and that mindset of 'shit if i don't study i'll get x and wont be able to achieve y'. As long as you know your content, there's nothing more you could do. Go in, kick ass and come out with results. In saying that, preliminaries are definitely the best time to figure all the exam techniques out.
End note: Enjoy the rest of your highschool experience. Definitely you'll enjoy uni more and 'real life' but high school is the only time you'll be able to be this close to anyone.