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

Sure! Just use one of your 5 re-take passes.  Just remember, you must use the passes BEFORE you take the final exam.  

You cannot use Re-Take passes on the final exam.

You cannot use Re-Take passes on any TST with a non-zero score.  

You get 5 re-take passes, no more.  Teachers are NOT able to issue more than 5 passes per student.