1) Consume a beet smoothie (with strawberries for taste) as your final meal before the fast, so you can judge what comes out when.
2) Refeeding after the fast is important. Make sure you read how to do it (vegetable juices and whole fruits for as long as possible after the fast). If your school starts on Jan 4, you should be at least two days into refeeding by then, so your fast should end no later than Jan 2.