The following are starting point for finding useful data sets for GIS assignments.
Fairfax County GIS and Mapping: Ffx. Co. offers applications, shapefiles, and maps for download and without cost. Examples of applications are the crime mapper that shows incidents by geographic location. Shape files include census tract data, elementary, middle, and high school attendance areas. The maps are both historical and current maps of the county.
CIA World Factbook: The world fact book offers downloadable .txt files of all of its country comparison data. This data can then be geo-coded in ArcMap. Students can compare debt, energy consumption, and military expenditures by region, country, etc.
The National Historical Geographic Information System (NHGIS): This has aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2000. You need to register, but it is not difficult and without cost. Most importantly is that this data is visualized via the Social Explorer.
Social Explorer: This is a very nice resource that allows for the creation, using a very simple interface, of census data driven maps that can be animated over time. There is a cost for the professional subscription. I have requested that information.
Gapminder: Gapminder is not a pure GIS application, however, it is a very powerful tool to demonstrate major geographic and social trends over the course of years, decades, centuries and even millenia. The videos are of Hans Rosling, who makes lecture topics like comparing the birth rate per thousand to life expectancy exciting and fun.