Thanks for all the great comments. This shoot was really a blast. I made 3 plates in less than 30 minutes, so it was pretty hectic. The first 2 were just my daughter and her date, then we added their friends for this last one. My daughter's poor prom date had no idea what he was in for when she told him "my dad wants to take our picture before leave for dinner". What you don't see is the crowd of parents and other on-lookers behind me adding to the chaos. There were lots of oohs and ahhs when they saw the results.
Yes, the swirly background is the result of using a Petzval lens on a larger plate than it was designed for. This lens was probably designed as a half plate or whole plate (6.5x8.5) portrait lens, not 8x10.