This is what I found:
Study results to date show that there were 173 incidents between July 1, 1994 and June 30, 1998. The majority of these incidents were homicides and involved the use of firearms. The total number of events has decreased steadily since the 1992-1993 school year. However, the total number of multiple victim events appears to have increased. During the past three school years, August 1995 through June 1998, there were an average of five multiple victims events per year. This is compared to an average of one multiple victim event per year in the three years from August 1992 through July 1995. Thus, while the total number of events of school associated violent deaths have decreased, the total number of multiple-victim events appears to have increased. Data collection ended with the completion of the 1997-1998 academic year.
Source:
http://www.cdc.gov/od/oc/media/fact/violence.htm