is all systems go. all right, jon karl there at the white house. jon, thank you. next tonight, the high school shooting that broke out as seniors were rehearsing for graduation. it happened in dixon, illinois. a fellow senior with the gun. and then, the hero officer. here s abc s alex perez. reporter: horrified students hugging in relief outside an illinois high school, after a tragedy is averted by a quick-thinking school resource officer now being called a hero. shots fired at dixon high school. shots fired at dixon high school. reporter: police say it began just after 8:00 a.m., when a 19-year-old student showed up with a gun and opened fire. shooting near a gym packed with seniors rehearsing for graduation. all i heard was something like, that sounded like firecrackers, saw some smoke. the popping scared everybody. reporter: school resource officer mark dallas immediately jumping into action, chasing the gunman out of the school. with shots ringing out through the h