Highland Park police responded to a report of a student with a gun at TrueNorth Educational Cooperative 804 in Highland Park just before 1:40 p.m.
A post on the city's Facebook page said no shots were fired, and no one was hurt.
Investigators said they don't think the child, who was taken into custody, intended to hurt anyone.
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A gun was recovered, police said.
Police said there do not appear to be any additional suspects or threats.
Out of an abundance of caution, police plan to increase patrols around all Highland Park schools.
School District 113 said the incident took place at North Shore Academy, which is operated by TrueNorth Educational Cooperative 804 and part of North Shore School District 112.
Red Oak and Sherwood schools were placed in secure status due to police activity in the area, the district said.