I always just studied the subjects I had on that particular day. e.g. on Tuesdays I would have a triple period on chemistry and single periods of literature and biology. So on Tuesdays I would study only those subjects. Its better to reinforce knowledge you learnt right away imo.
I normally just left assignments for the weekends, which was a lot of time considering I didn't have a part-time job.
In terms of concentrating on the harder subjects rather than the easier subjects...never did that.
Even though further was the easiest for me I still weighed it as equal as the harder subjects like chemistry. The fact that it was easier just meant that it took less time to finish the homework and a bit extra, which ultimately would lead to more time for the other subject I had that day.
Anyways that was the way I studied in yr 12, good luck
