Don't know how far you are from St. Louis but it's 11 hour drive to New Orleans from there (I've been planning a family reunion cruise and many relatives live in St. Louis) and the Carnival Holiday and RCI Grandeur of the Seas sails from there. The Holiday does 4 and 5 night cruises and the GOS does 7 days. Either one of these ships would be great for your group. And New Orleans offers it's own entertainment for pre- or post- cruise vacation. At the size of your group you might get a couple of cabins for free.
But for me............spring break on a Carnival or RCI ship is scary. Ooooooh, that might actually be something to see if you're not the responsible party ! !<vbg>