Making casts of Animal Tracks is an old favorite for all ages, and completed casts offer a wonderful tool for studying tracks and an interactive addition to your classroom's natural history collection. Deep impressions in mud or sand are some of the easiest to cast. Teachers may prefer dentstone to plaster of paris for its durability, attractiveness, clarity of detail, and the fact that it works in wet conditions (dentstone is a material used by your dentist, so ask her for a free sample, or order it in 25 and 50 lb quantities from places like here or any dental supply company).
Just mix plaster or dentstone with water to the consistency of pancake batter, and pour to overfill a track well on all sides.
Placing a ½" - 1" high circular ring around each track or set of tracks before casting will help ensure adequate coverage and give an even outside shape to the cast. A plastic ring cut from the top of a yogurt container will work, or, for reusability, cut large diameter PVC pipe into rings. Cut a slit in the circle ahead of time in order to make cast removal possible. Mound dirt or sand around the outside of circular ring to prevent leakage. Collect the cast when it is completely hard, an hour to a couple of days, depending on conditions.