Appalachian School of Law, unarmed students subdued a shooter by using brute force. Apparently, the students may have been unaware of the presence of armed policemen.
http://en.wikipedia.org/wiki/Appalachian_School_of_Law_shooting

Regarding the frequency of school shootings, I have noticed that since Virginia Tech the frequency of school shootings has gone up noticeably. There may or may not be a correlation. There have been 35 American schools that had shootings between 2007 and today 10/25/10 (less than 4 years). In comparison, from 1966 to the end of 1983 (18 years), there were 9 American schools.
http://en.wikipedia.org/wiki/School_shooting

Those stats do not include other shootings outside of American schools. Also, I'm about certain those stats don't include gang-related shootings at schools.