(Olympia, WA)  --  State officials say public-school students across Washington are missing classes too frequently.  The state Office of Superintendent of Public Instruction says 16 percent of all Washington students missed at least 18 days of school last year.  Superintendent Randy Dorn told KING-5 that chronic absenteeism is a barrier that stands in the way of learning.  The department says Native American, Pacific Islander, and low-income students had the highest percentages of chronic absenteeism in 2015.