"It was crazy. It was like killers on the loose," said Joshua Glaze, 15, a freshman at the school.
The game was in overtime and some students began to leave the building, Glaze said.
Dunbar freshman Shaneisha Turman, 15, said she saw a silver truck pull up to a bus stop outside the school. People inside the truck pretended to fire a gun and then "they really started shooting," she told the Chicago Tribune.
People were shouting, "'They're shooting!"' Turman said. "It was a lot of people shouting and running."
No one was in custody, said Chicago Police Department spokeswoman Antoinette Ursitti.
Police have blocked access to streets at least two blocks from the high school, and several ambulances and police cars are at the scene.
No one answered the telephone at the school.
