When I taught 7th grade language arts, my students had a horrible time inserting commas in their correct locations. Scroll back about 40 years to my freshman college English comp class - the professor warned us that he did his doctoral thesis on the comma. Say what?? We took that as a warning. I never had a misplaced comma again!
So, I developed a relatively easy way to remember where commas go - this is the abbreviated elementary version so young authors can understand the intricacies of comma usage! I hope this helps your students, as well.
(OK, I sorta stole Culture Club's "Karma Chameleon" for my image - but I bet you're singing that in your head right now!)