Prioritise what you struggle with first. As previously stated by other users, if you are acing a subject but barely passing another, it doesn't make sense to maximise what little extra mark in a subject which you don't struggle with. If there is something key that's worth remembering, then for sure spend a while trying to remember it, whether it be a definition or a basic concept or writing style.
If you want to get ahead in a subject, then do some study before the next topic. Sparknotes.com is a place I frequently use, and if you scour the forums a bit, you'll find plenty of other great resources as well.
If your school offers its own online resources, then visit that portal and find some practice SACs, revision material or photocopies of textbook information to read up on, as it has already been curated by your teachers and will be found useful at some point during the year.
If you have the time, then visit the VCAA website and find past exams and try to answer some questions on it. Use past exams as checklists to see what you already know, what you should revise and what you will need to know later on.
Teachers don't necessarily offer the best advice on study, and especially when speaking to a class what methods they suggest for study can be really generalised and won't work as great as they claim to. However, they don't make those kinds of suggestions for no reason, there is rhyme and reason behind what they are saying, so you may use their advice and bend it to what suits you. If the idea of studying a subject per day sounds like it could be useful if implemented more personally, then make yourself a timetable and plan out what exactly you want to do.
An idea I may try in the near future is to create a to-do list of small dot points in every subject for material that I believe is important, or something that I don't quite understand. That way when I have the time to do study of my own I'll have a full list of concepts and ideas to read up on.