A classroom pond holds all the stuff and drama of life. Peaceful plants, stately snails, wiggling tadpoles and darting, swimming fish. Any large and wide container can work as a pond. You want to make sure it is sturdy. For large classrooms a child's wading pool works great.
Smaller classrooms can use tupperware sweater boxes or wide and low planters.
The water you fill it with should be treated with a water treatment product from the pet store that takes out chlorine and chloramines. Aquarium gravel should cover the bottom of your pond. The most important thing about a classroom pond is that it be near a source of light. A window is the very best option. If there is no window in the classroom, the next best thing is artificial light.