If it isn't a fish or polliwog chances are it's an insect. Put any insects you catch in a jar with some water. Draw or write about the insects you caught. Look them up in your field guide. Chances are that many of the insects you see in the water are larval stages of the insects you see on land and in the air.
Many insects stat their lives in water and then move out of the water as they mature.
Take the insects you caught back to the classroom and try to identify them more clearly. See what you can learn about them. Before a rain, look at your school grounds. Put markers where you think water will gather. After the rain, do check out your hypotheses.