This article deals with the isolation, fixing and staining of bacteria. I would not recommend the use of some of the solvents that they use (such as xylol) with children, however. They also describe a blood smear preparation, what I do not recommend for schools (it may not even be allowed in some countries). Still, the article gives a very nice introduction into several preparatory techniques. The article stretches over several pages, click the link at the end of the pages to continue reading. The fact that the article was published 75 years ago, in 1934, does not matter. The preparatory method stayed the same.
Link to the article: Microb hunting with your Microscope (Popular Science, Sept 1934)