Not knowing where the classes are could be a problem for maybe, like, the first day and that's it. Chances are you'll have a timetable that will tell you where to go and when and you'll maybe have been shown around the school before.
Basically you arrive in the morning, you have timetabled lessons but with empty study periods at certain times in which it's entirely up to you how you spend that time. Most schools/6th form colleges will have a common room where you can relax or work when you don't have a lesson.
By the way, are you only doing two A-levels? People usually do three...