The Rainbow/PUSH Coalition leader addressed the city's "no-snitch" culture at an assembly at King as police continue the search for the gunman who fatally shot student Hadiya Pendleton not far from the school Jan. 29.
"The price we pay is that the killer goes free and has the option to kill somebody else," he said.
He then asked the students, "If today you knew a friend had some drugs or a gun in their car outside, would you tell it?"
The crowd murmured "no's" in response, and when Jackson repeated his question, their refusal was stronger.
When Jackson asked if they would turn in someone with a rope and hood in their car — a member of the Ku Klux Klan — they shouted "yes!"
"We lose more lives from bullets than from the rope," Jackson said. "You're willing to turn in the Klan member that does not exist in your life, but not turn in the threat that sent you to a funeral this coming week."
Student Jeffrey Faulkner said Jackson's words put the "no-snitch" culture in perspective for him.
"It changed my thoughts on what I would do," he said. "If something happened to someone I know, I would say something.
Ashley Poindexter, a freshman at King who was on the volleyball team with Hadiya, said students needed to hear Jackson's words.
"We needed someone to talk to us and inspire us, someone who has seen it before," she said. "It made us understand that it doesn't matter what the situation is. You need to step up and help Chicago. We need to make a statement."
Jackson had called for President Barack Obama to attend the funeral for the slain 15-year-old last week.
Hadiya was killed while hanging out with friends at Vivian Gordon Harsh Park after taking her final exams. A gunman emerged from the alley and hit Hadiya in the back. Another student was shot in the leg.
A $40,000 reward is being offered for information leading to the arrest of the girl's killer.