Currently, the state uses a 4 - year graduation rate calculation that based on the calculations recommended by the National Governors Association. (aypf.org)
Graduation and Dropout Statistics for Washington's Counties, Districts, and Schools Graduation Rate Calculations - March 2012 (PDF) (k12.wa.us)
Except that students who leave the school before 12th grade apparently don't get included in the graduation rate calculation at least for schools like BASIS, as each BASiS school is reporting a 100 % graduation rate or something very close to that according to U.S. News and World Report. (grandcanyoninstitute.org)