Different kinds of nutrients in foods feature in all key stage 3 courses and students learn ways to test for carbohydrates, proteins and fats. Add a qualitative test for vitamin C. Use methylene blue or DCPIP as an indicator - they are both decolorised by the vitamin. Crush chicken bones (or use bone meal from the garden centre) and use the flame-test technique to show that there is calcium in bones.
For a comparative study of ascorbic acid in different "juice drinks", use DCPIP in a quantitative way. Find the volume of a drink needed to decolorise a known volume of indicator and compare this with the amount of a known concentration of ascorbic acid needed to decolorise the same volume of indicator. Look at evidence on vitamin deficiency and health. Longman Biology by Williams and Workman features James Lind and the link between scurvy and vitamin C.