The maps pages and education pages of the USGS should be bookmarked by anyone who teaches geography. One of my go-to pages within the USGS education site is this collection of 27 ideas for teaching with topographic maps. In the list of lesson ideas you will find suggestions for lessons about typical geography topics like coordinates, scale, and map projections. The USGS offers a free map projections poster that you can use in conjunction with the lesson on map projections.
You can download hundreds of USGS maps for free from the USGS store. You can also visit the USGS topoView site to download historic maps. One of the ways that I like to use these maps is to overlay them on Google Earth imagery. This can be a good way to compare map projections and map content. This video shows you how to do that.
Projection Wizard is an interesting tool developed by Bojan Šavrič at Oregon State University. The purpose of Projection Wizard is to help cartographers select the best map projections for their projects. Projection Wizard is a more advanced tool than most high school geography courses would need. That said, I would use the Projection Wizard to have students discuss the flaws of various map projections. We’d also talk about why a particular type of projection is better than another for different types of projects.