It happened in the street outside; the gunman never attempted to enter the school.
https://abc7chicago.com/post/berwyn-police-outside-lincoln-middle-school-6432-16th-street-video-shows-car-crash/17829899/
Berwyn police officers heard shots ring out just after 4 p.m. and rushed to the scene at 16th and Gunderson. There, officers saw a male suspect, in a vehicle, firing a rifle into another vehicle.
*snip*
"Upon arrival, officers observed a male suspect firing a rifle into another vehicle," said Berwyn Police Division Commander Michael Fellows. "As officers approached the scene, they came under fire from the suspect. Our officers took immediate cover but did not return fire."
The shooting prompted a hard lockdown at Lincoln Middle School, where some students and staff were still inside, just steps away. A windshield was left covered in bullet holes.
"snip"
Police said once more support arrived, officers approached the suspect's vehicle and found him dead after he apparently shot himself. Officers also found two other people dead inside the vehicle that he was shooting into.
So, no kids were involved, thank goodness. There weren't many kids or adults in the school at that time of day, but the school (wisely, imo) immediately went into lockdown at the sound of shots being fired outside. School is also closed today (not sure about the rest of the week).