The attorney for Falandez James says James killed 17-year-old Carron London in self-defense after the teen's gun misfired during a confrontation outside a Coatesville gas station in June 2010.
Prosecutors say London tried to fire at James after being shot. London was a student at Coatesville High School.
James is charged with first- and third-degree murder in the killing and could face life in prison if convicted.
The trial in Chester County court is expected to last through the end of the week.