Fruit, cookies and lettuce grow mold well, but the undisputed king of mold is bread. Stick a slice of bread in the right environment and you should have a nice, healthy mold garden within a few days. One method is to seal it in a see-through plastic container. Put the container in a warm, dark spot and let nature take its course. Within one or two days you should see your mold garden proceeding very well.
A similar method of growing mold would be to slip a slice of bread into an airtight bag. A freezer bag with two sealing locks is best. Much as you would with a plastic container, set the bag in an environment that is ripe for mold: warm, dark and moist. Check back periodically. In the outdoors, mold breaks down dead stuff like leaves, plants and other debris by feeding on the material. It will do the same with your slice of bread.
A critical aspect to encouraging mold to grow quickly is to introduce moisture into the environment. It doesn't have to be a lot of moisture. One method that works well is to hold your fingertips under the water faucet just prior to sealing the bag or plastic container with the bread inside. Wipe your fingers on the bread or let a few drops drip onto it, and then seal it and place it in your chosen spot. With whatever method you choose, it's critical to leave the bread exposed to the open air for about half an hour before closing it up. This allows stray mold spores a chance to accumulate.
An interesting fact to note is that mold is a common source of allergens and can make you sick. It's probably not a good idea to stick your nose in the container or bag and breathe deeply at the conclusion of the mold growing session. Whether for a school science experiment or your own education, you now are equipped with the tools and knowledge to grow a healthy collection of mold using nothing more than simple stuff you probably already have around the house.