When you first log into SIMS you be presented with a large view of your timetable. Any classes that you have been assigned to cover should update on your timetable here. You can swipe left and right to see your previous and future week's.
If you tap on one of your classes, you'll then see a list of your students in that class. You'll also notice their photographs will appear, which is particularly handy when you have a new Year 8 class or are covering for somebody else!
If you need to, you can always tap your name on the top right of SIMs to search for another teacher's timetable. This will then let you see their timetable and classes, but it won't show you their marksheets.