Complex sentence: School shootings are rare and unusual forms of school violence, and are accountable for less than 1% of violent crimes in public schools, with an average of 16.5 deaths per year from 2001--2008.
Split into simpler sentences: School shootings are rare and unusual forms of school violence. They are accountable for less than 1% of violent crimes in public schools, with an average of 16.5 deaths per year from 2001--2008.