If a student sits the Placement Test and is on Level C, and then throughout the year they complete the lessons and want to re-sit the Placement Test, would you then re-set the Placement Test for them?

Generally, the student should be moving up to D as they have completed Level C. After they have completed their STARS lessons in C you would be able to assess their mastery with the Post Tests. Based on these results, you would be able to move them to the higher level without having your student sit the Placement Test again.