On the last day, if you want to go out with a bang on Graduation Day. be sure to buy a surplus amount of instant action laxatives.
Try to get around inside the tuckshop/canteen without being noticed and mix it all with the tamato sauce, or a similar type of sauce (not to thick) and make sure it blends in really well. Then sit back and watch all the kids rush for the toilet.
It goes without saying that the school janitor will have a hard week ahead of him.