Nearly one in five students entering ninth grade in Suffolk’s public high schools will drop out before graduation, according to the results of a statewide study of on-time graduation rates.

With a dropout rate of 18.6 percent, Suffolk students are more than twice as likely to leave school early without a diploma as the state average. Fewer than three-quarters of the city’s public-school students finish high school on time, compared to the 82.1-percent average in Virginia.

The numbers represent the first time that the commonwealth has tracked students from ninth grade through their scheduled graduation date, giving educators a “cohort report,” instead of just snapshots of classes at graduation time.

Earlier methods of calculating graduation rates often failed to account for students who completed alternative programs, moved to other school divisions or just took longer than the normal four years to move through high school.

“The publication of these cohort reports represents a milestone in the commonwealth’s effort to account for every student,” Virginia Superintendent of Public Instruction Patricia I. Wright said in releasing the report. “This is vital information that will shape efforts at the state and local levels to keep students in school and on track toward earning a diploma.”

Suffolk School Superintendent Milton R. Liverman and Kevin Alston, assistant superintendent for administrative services, were both unavailable for comment on Tuesday.

The study showed that of the 1,106 students in Suffolk’s “cohort,” 798 received some kind of diploma within four years. Virginia recognizes and awards an Advanced Studies Diploma, a Standard Diploma, a Modified Standard Diploma, a Special Diploma and a General Achievement Diploma for students of varying academic abilities.

Another 12 students from the original group received either a General Educational Development certificate or a Certificate of Completion. Forty-six students are still enrolled in school after four years, and 206 are listed as dropouts.

Among the eight cities and counties comprising Greater Hampton Roads, Suffolk placed ahead of only Portsmouth in its dropout rate. The percentage of students who left school early ranged from a low of 5.5 percent in Virginia Beach to a high of 19 percent in Portsmouth.

Portsmouth also found itself at the lowest rank in on-time completion rates, with just 61.4 percent of its students finishing high school in four years. With an 87.4-percent rate, Chesapeake led in that category.

Among the study’s conclusions are that students who repeated grades, attended multiple schools or were frequently absent were more likely to drop out.

Absenteeism turns out to be a leading predictor of whether a student will drop out of school; 65.2 percent of dropouts had attendance rates lower than 80 percent during their final year of school.

Nearly as many, though — 58.8 percent — had repeated at least one grade during high school. More than a third of those who had to repeat the ninth grade ended up dropping out.

“Using the data from these cohort reports, educators and policymakers can now see where interventions are most urgently needed and identify high schools and school divisions that have developed best practices and strategies that others can emulate and adapt,” Virginia Board of Education President Mark E. Emblidge said.