"Other recent shootings at schools" summary

A lot of shootings occur at schools in United States every year. Young adolescents from 6 to 19 years old use guns, wound and kill innocent classmates and teachers.
Some of criminals were mentally ill, but some of them were just very cruel and not responsible of their actions and maybe wanted to revenge somebody for something. Most of murderers were convicted, some of them were shot by police officers or killed themselves, but those,who were sentenced, got different punishments.
American teenagers are very violent without logical explanations and US's government must take some measures to stop this horror.