OSU gives students a lesson on debt [The Columbus Dispatch via The Quick and the Ed] - No reason you can't do this in some form for K-12.
Free Credit Lesson Plans for Middle School and High School Teachers - Financial education is both a necessity and a no-brainer as far as making math more relevant for our kids.
Elementary Math, Reading Skills At Age 7 Linked To Financial Success At Midlife, According To Study [HuffPo] - Perhaps the best financial literacy we can teach is indirect: making sure our children can read and write fluently in elementary school.
Use This "Bank Ledger" to Manage Your Kids' Allowance [Lifehacker] - This one is meant to be used at home, but I think it could be adapted to an early elementary classroom using play money or something of the sort.
7 Places Teens (and Adults) Can Learn About Money [Wise Bread]