As long as you keep focused, it will be doable. Speaking as someone who was performing 4 shows per week for a combined 12 weeks of my AS year (with rehearsals throughout the year), and went on to get 4 As at AS, you just need to stay calm and work away at your subjects. Start revision as early as possible - 3/4 hours of extra work (including homework) for each subject should be sufficient if you're working throughout the year (although when it comes to actual exam period, you'll want to significantly increase this). If flashcards help, make them and always carry a set with you, and test yourself on the bus home. Make a revision timetable (I used getrevising for this) and stick to it. If you get frees, make use of them, because they significantly reduce how much you need to work at home. If you need any more help, I'm more than happy to lend a hand!