Two Republican senators Tuesday requested a study of a long-controversial part of Florida’s school-funding formula, arguing it helps urban school districts at the expense of suburban and rural districts. Senate Education Chairwoman Dorothy Hukill, R-Port Orange, and Sen. Travis Hutson, R-Elkton, sent a letter to Senate President Joe Negron, R-Stuart, seeking a study of what is known as the “district cost differential. That part of the school-funding formula takes into account costs of living in different parts of the state and has repeatedly spurred legislative debates over the years.