It depends on the equipment you have available at your school. Some have sensor probes that can detect the amount of CO2 in the air (useful for plant experiments), others may have chemicals that change colors depending on the amount of CO2 dissolved in some water (useful for algae experiments). Assuming you have access to one of those, you could do experiments where you measure the rate of change in CO2 amount over time (maybe varying how much light the plant gets). You could do an experiment with algae where you change the amount of dissolved CO2 in the water (there are various chemicals that will lower it, and you can add baking soda or even breath out with a straw into the water to spike the CO2) and then measure the rate of photosynthesis. One lab has students use a common aquarium plant, elodea. If you put it in upside down and cut off a section of the stem underwater, it'll release bubbles of O2 as it does Photosynthesis, count bubbles to measure rate.