The U.S. Department of Education used a common metric to calculate graduation rates for all states and the District of Columbia for the 2010-11 school year -- the first time it has done so. The statistics, released Monday, show the rate at which first-time ninth-graders earn a high-school diploma within four years. Arkansas had the highest graduation rate for students with disabilities at 75%, while Mississippi and Nevada had the lowest at 23%. Many states reported lower graduation rates than revealed in past calculations, and the data also show gaps in achievement among students.