Witnesses said the shooter, who was dressed in black, burst into the lecture hall with more than 100 students inside and began randomly firing a shotgun.
Students, some of them bleeding from neck and other wounds after being hit, fled the classroom.
George Gaynor, a geography student who was in the hall when the shooting happened, told the Northern Star student newspaper that the shooter was "a skinny white guy with a stocking cap on".
He described the scene immediately following the incident as terrifying and chaotic.
"Some girl got hit in the eye, a guy got hit in the leg,'' he said.
Kishwaukee Community hospital said on its website that up to 17 victims of the shooting were arriving, including three to four with head wounds.
A hospital spokeswoman said the injured had been shot or were hurt by flying debris, and that three had serious injuries.
All classes were cancelled on Thursday night and Friday at the 25,000-student campus and students were urged to call their parents "as soon as possible''.
"People are urged not to come to campus," the university's website said.
The shooting comes 10 months after 32 students and staff were gunned down by a student at Virginia Tech University in the deadliest massacre ever at a US educational establishment.
Source: Al Jazeera and agencies