Some parents had to wait up to 5 hours for their children to be dropped off at a nearby bus park, according to local media.
Wisconsin state attorney general Josh Kaul said at a press conference on Wednesday that no other students or employees were injured in the incident.
Police said only that the student who was killed was male and did not say what kind of weapon he came to school with.
"This kind of incident is a parent's nightmare," Kaul said, adding, "There was a report that someone came to school with a gun and law enforcement went there. This is a very dangerous situation."
The school where the incident took place will be open on Thursday and the affected staff and students will receive moral support, the statement said.
