Is there a way you can test potentially unclean water? (But not something really expensive)?

Fake out your chemistry teacher by using benthic macroinvertebrates, the critters that live in the streams and are ociated with the bottom. The greater the diversity, the cleaner the stream. If the stream is dominated by fly larvae, sewage worms, snails, and flatworms, the stream has been overfertilized, possibly from all the lawns. If it is dominated by water pennies, mayflies, and stoneflies, the stream is "clean". You don't need much, if any, magnification, unless you are trying to identify something like midge larvae down to species (a really tough job). Talk to your biology teacher about "biomonitoring".