I find Water Fasts to be a rollercoaster of all sorts of stress. It is advisable to set aside some time when you do not physically/mentally need to exert yourself. I find sufficient amount of rest and even a small amount of physical activity can either make one feel energize or just zapped out. It all depends on the intensity your body is detoxing. Sometimes you will have a burst of energy and the next you feel like garbage. There is no way around it. Except maybe juice fasting. My personal suggestion would be to juice if you are concern about your energy levels for school.