Tests that are missing include - tests of the functionality we talked about elsewhere, namely:
* Set due = Monday, extended_due = Wednesday, assert that what should happens happens (latest day taken)
*  next, due date is changed to Friday. Assert again that latest day taken

I'd also like to see a test that tests making quite a few students, sets different late days for some of them (student 1 due Tuesday, student 2 due Wednesday, rest of students due Monday AND rest of students due Thursday), and make sure that every individual student ends up with the right day.

Again I'll note: to mock out students please use `UserFactory`. To mock out staff or instructor use `StaffFactory` and `InstructorFactory`, respectively.