Violence in the classroom distracts students' attention -- and in many cases so jars them that their mental performance is injured for days afterward. In a new research paper, economist Jeffery Grogger of the University of California at Los Angeles studies the impact of school violence.

• Using nationwide public high school survey data, Grogger found that minor levels of violence -- a problem faced by nearly two-thirds of public school students -- lowered the chances of students graduating from high school by about one percentage point, to 78 percent.

• It also lowered their chances of going on to a four-year college by four percentage points -- to 27 percent.

• Moderate levels of violence -- faced by 9 percent of students in the sample -- reduced the likelihood of high school graduation by about 5 percentage points and of college attendance by 7 percentage points.

The effects of more serious violence were, of course, even greater.

The impact is felt not only in terms of students' attention span, but in school days lost through fear.

Source: Gene Koretz, "The Class of Boxcutter High," Business Week, August 11, 1997.