You firstly need to de-starch a plant by storing it in darkness for 24-48 hours. This obviously needs to be done well in advance of the lesson.
Geraniums work well for most plant practicals in biology, It is worth looking after several plants at school rather than bringing one in each time one is required.
The plant is placed within a plastic bag and a small open pot of soda lime is placed in the plant pot. The bag is sealed so that air cannot get in or out. Make sure the bag is free from holes before you begin.
The plant is then left in bright sunlight for several hours and then a leaf removed and tested for the presence of starch. For details of how to test a leaf for starch, click the link at the bottom of this page.
Through testing, students will see that there will be no starch present, proving that photosynthesis cannot take place without carbon dioxide.
It may be worth using a control plant (one that is not in a plastic bag) which can be tested for starch alongside the other plant for comparison.