End of semester

What happens at the end of a semester? Can you start over with grades for the same class, but keep the data from the previous semester for reference? We are required to keep grade book data for a year.

Yes, you can do this at different levels

At grade book level:
I'd suggest to simply hide the tabs of your previous semesters , tap&hold on a tab, then Hide
You can review a hidden tab from the Tools icon on the top bar, Tabs.
If you'd like to reuse the same tab's structure for for the new semester, tap&hold on the original tab , then Copy / Columns

At class level:
If, on the other hand, you'd like a brand new grade book and diary (which will require you to update your schedule). You can hide your last semester's class, and create new one.
In order to hide a class, from the main screen , tap&hold on it , then Hide.

You can start a new class with previous semesters' students, and grade book's strucutre.
Tap&hold on your original class, Copy and select 'Students and columns' .