According to the Montgomery County Public School System:

The following information was sent to us by Rockville Public Information Office:

The graduation rate for Montgomery County Public Schools (MCPS) students remains strong and is higher than the rest of the state, regardless of the formula used to calculate it. 
  The Maryland State Department of Education calculated the graduation rate using two different formulas this year as part of the annual Adequate Yearly Progress (AYP) report for high schools. The graduation rate was calculated using the “leaver rate,” which has been used for the past decade, and the new “cohort rate,” which is being used for the first time. 
  Under the leaver rate, the graduation rate for the class of 2011 was 90.7 percent, slightly higher than 2010 (90.1 percent) and higher than the state’s 2011 leaver rate (87 percent).