This is proving to be an interesting read. Direct from secretservice.gov. http://www.secretservice.gov/ntac/ssi_final_report.pdf

Despite prompt law enforcement responses, most shooting incidents were
stopped by means other than law enforcement intervention.


...

Most school-based attacks were stopped through intervention by school
administrators, educators and students-or by the attacker stopping on his own. In
about one-third of the incidents, the attacker was apprehended by or surrendered to
administrators, faculty, or school staff (27 percent, n=10) or to students (5 percent,
n=2).


...

Just over one-quarter of the incidents were stopped through law enforcement
intervention (27 percent, n=10). Law enforcement personnel dischar ged weapons in
only three of the incidents of targeted school violence studied (8 percent, n=3).