Can I retake a quiz/test that I've already completed to try and get a higher score?

Sure! Just use one of your retake passes.  Just remember, you must use the passes BEFORE you take the final exam.  

You cannot use retake passes on the final exam.

The score of your most recent attempt is recorded, NOT the highest score of your attempts.

You get 10 retake passes, no more.  Teachers are NOT able to issue more than 10 retake passes per student.