If you want to show your students the religious make-up of America, you should check out these awesome maps from from the Washington Post.
The map above shows the largest non-Christian groups in different regions. You'll see that Islam is the largest, especially in the midwest and the south.
Other maps will show you which counties have the largest number of adherents, which have the most religious participation, and which have the most religious diversity. Another map shows the total number of congregations as a share of the population. My thanks to F.C. Tymrak for tweeting the link.