John, John, strong and ableLooking it up, the last line has minor variations, and you were supposed to zip in the name of the offender instead of "Mabel" in the first line. It seems to have been used at summer camps a lot, but we had it at home. I never got the part about people putting their elbows on tables in barns - children are very literal that way. I got the point that it was supposed to be ill-bred and not refined, but I still thought stables must actually have something to do with it somehow. Couldn't figure out why, as stables didn't necessarily have tables at all. But if they did, you stood at them instead of sitting, and were doing some unclean work, and your elbows might touch that table...
Keep your elbows off the table
This is not a horses' stable
But a proper dining table.
In browsing, I found a wonderful Etiquette Is The Height Of Rudeness post.