Prioritising is key. So you get home from school and you have a bunch of homework, some exams and assignments coming up. Break everything down by their relative importance. Personally, I think homework should be done first; you don't want to fall behind in it. Many people underestimate the importance of homework and do not consider it as a form of studying. But if you're up to date, your exam study will become much easier.
After homework, I think consolidating what's been done in class + homework is essential. Spend 10-15 minutes each day writing up notes for your subjects. This isn't formal study but means that when you have to start revising for exams, your notes will be up to date so you can focus on learning them. Only revise notes if you have the time; I think that keeping up to date with homework and notes is much more important.
With exams, give yourself plenty of time to familiarise yourself with what will be examined. I know this is easier said than done, but starting early makes things a whole lot easier. Personally for maths exams, I'd just stay on top of my homework, consistently make short notes on theory and do practice questions periodically. This meant that by exam time I didn't need to heavily study, I'd be doing past papers to consolidate.
With time management, there's no easy answer. People work differently so different strategies are going to suit different people. Making a timetable is useful, but I will tell you that it sometimes doesn't work. Because some days you'll get heaps of homework for a subject that's not listed on your timetable etc. So what I did was make a table of all my subjects and tally everyday I did 1 hour of study for that subject (this includes homework, making notes, revising). So I didn't have a strict timetable, but I could see if I was spending too much time on a certain subject. This really helped me spend equal time on all subjects.
I also recommend switching between subjects regularly. Personally I don't have a large attention span so I couldn't spend a long time on one subject. So do an hour of one, then change. If you can go for longer with changing, good but don't be afraid to leave a subject and go back to it later.
All in all, if you chip away periodically at your work and consistently study in small doses, you'll do very well. It's all about doing stuff now rather than later. Good luck