Juulie - If you do not already have accesses to the 3 videos on the following link have your board get them. One can talk until they are blue in the face about block/charge and traveling but a video of the violation is worth 10,000 words.
http://www.nfhs.org/catalog/pubcat02a.pdf
Try to schedule a game were you have experienced refs calling it. Have your class in the stands for the first half and ask questions on why they think X was called and Y was not. For the second half have them on the floor behind the bench so they can see what a coach is seeing.