In years 12 and 13 your personal timetable is entirely shaped by what you've chosen to study at A level. Every timetable also includes some free periods so that you can get on with private study. If there's a free period in your timetable at the end of the day, you may be able to go home early.
Each AS-level course requires about four and a half hours of private study per week and each A2-level course requires about six hours.