The Snowy Mountains Hydroelectric scheme is a very large example of how flowing water can be turned into electricity. Once you get your head around the idea of a turbine, the idea is relatively simple. Here are some videos about hydro power and this particular scheme...
So how can hydro power be useful in the classroom? Here are some domestic examples to give you inspiration. Maybe try popping a turbine in a sink...
Free electricity from running water sounds great, but can you really power your house just by washing your dishes? Here is a video that does the maths...