Photosynthesis is easily demonstrated in the classroom using the freshwater aquatic plants Elodea or Anacharis, which can usually be purchased inexpensively from an aquarium shop.
Set up several pairs of test tubes, so that each small group of students has two test tubes to work with. Have them fill each test tube half full of phenol red solution. Phenol red is a pH indicator that turns red when its pH is 7 or more (the alkaline range) and changes to a pale yellow when its pH is less than 7 (the acid range).
Have two students in each group use straws to blow air into their two test tubes until the phenol red turns yellow. They are adding carbon dioxide to the solution, which makes it more acidic.
Next, add a sprig of Elodea or Anacharis to one of each pair of test tubes, leaving the other test tube with phenol red solution alone. Place the test tubes under lights (a desk lamp will do) for 15 to 20 minutes. The Elodea rapidly begins to photosynthesize, using up the carbon dioxide in the water. There should be a color change in the phenol red solution (it turns red again) after the plant has used up enough carbon dioxide to raise the pH. Another indication that photosynthesis is occurring can be seen by the tiny bubbles of oxygen that form in the test tubes containing the aquatic plant sprigs.