Evanston fire and police units responded to Nichols Middle School this afternoon after some students on upper floors of the building reported difficulty breathing.
The problem at the school was first reported shortly after 3 p.m.
Fire officials responding to the scene reportedly suspected there might have been a gas leak in the building but were unable to locate any actual leak.
They ventilated the third and fourth floors of the building and now report no sign of gas fumes in the building on their test instruments.
At least three students were treated at the scene by paramedics for shortness of breath.