I saw this homemade sprinkler highlighted online and had to try it myself to be used for water day during Beach Week.
Use an empty 2-liter soda bottle. Drill small holes in the sides of the bottle at various spots around the bottle. Attach the water hose with an adapter on the end of it into the mouth of the soda bottle. Turn on the water. Once the bottle fills up with water, then it starts to squirt out of the drilled holes.
This sprinkler will make a great water day for the younger preschool students (toddlers-2 year olds), but the older preschool students will enjoy it as well.