http://tlt.psu.edu/suggestions/cases/links.html
Penn State has a website dedicated to using case studies in teaching. On the home page (see link above), you will be provided with links to pages teaching you what a case is, how to write one, how best to utilize cases in the classroom, and others. There are also cases available free on the website to use in your classroom, broken down by subject area. The only drawback to the site is that it is geared toward upper-level education, high school and beyond, so if you are a middle- or elementary-school teacher, you may not find many examples that would work for you.