You can look at material in your local pond. I would recommend placing a drop on a slide, use black paper below the slide, and side illumination (see How to Use QX5) with a LED flash light. Mosquito larvae are also easy to see and found in stagnant spring water.

Carolina also sells Pond Mystery Mix (Cat. 13-2010A). One of two will be enough for a whole classroom. This is dried material and spores germinate after adding water (ideally Carolina Spring Water (Cat. 13-2450)); swimming creatures will be visible in a week.