CHICAGO — A comprehensive study released Thursday has finally proven what anecdotal evidence has long suggested: Poorly qualified teachers drag down student achievement.

The first-of-its-kind study demonstrated that elementary and high school students — even those in middle-and upper-income families — post higher scores on state exams and are more prepared for college if they attend schools where teacher quality is ranked high.

Low-income and minority children benefit the most from good teachers, the study found.

In Illinois' poorest elementary schools with low-teacher quality, the average pass rate on state tests was 31 percent. But in similar low-income schools with higher-ranked teachers, the rate jumped to 43 percent, research revealed.

The researchers evaluated teachers in Illinois, Wisconsin and Ohio, ranking schools according to a teacher quality score.

In Illinois, that score was determined by five factors: the average college entrance exam score of all teachers in the school; results on the teacher licensing test of basic skills; a national ranking of college attended; years of experience; and number of teachers with provisional credentials. All of the state's 3,800 public schools were evaluated.

Chicago Public Schools fared particularly poorly in the study, with three-quarters of the campuses landing on the bottom of the pile in teacher quality. Individual school scores were not released.

"We now know that all kinds of kids, poor, rich, minority, white are affected by their teacher's ability," said Kati Haycock, who heads Education Trust, the Washington think tank that helped underwrite the study. "The research shows that kids who have two, three, four strong teachers in a row will eventually excel, no matter what their background, while kids who have even two weak teachers in a row will never recover."

The most startling findings in the report relate to the link between teacher quality and student performance, especially when it pertains to low-income and minority students. And the negative effects multiply the longer children remain in schools with low-performing teachers.

In elementary schools where more than half the students were low-income, an average of 44 percent of students passed math, reading, writing, science and social science exams when the school was filled with low-quality teachers.

In poor schools with better teachers, the pass rate jumped to 56 percent.

Even in richer schools, kids performed better if they had better teachers. On average, about 78 percent of students in the highest-income schools with the worst teachers passed state exams. In similar schools with better teachers, the rate jumped to 84 percent.

The achievement gaps grew significantly larger in high schools.

In low-income high school where the teacher quality was ranked in the cellar, the average pass rate on state exams was about 14 percent. But in poor schools with a better teaching corps, 33 percent of students passed state exams.

In high schools where the teachers had the lowest qualifications, only 15 percent of students were prepared for college, compared with 50 percent in schools where teachers were more qualified.