Buffalo Grove police officers responded to the crash at Buffalo Grove High School around 3:20 p.m.
Police said a driver was trying to exit a school parking lot onto Dundee Road when they left the roadway and hit a pole, which then hit the staff member.
The staff member was transported to a local hospital in critical condition.
Dundee Road was closed for over three hours, and the driver is cooperating with police.
The Buffalo Grove Police Department Traffic Unit and the Major Crash Assistance Team are investigating.
In a letter to parents, Buffalo Grove Principal Jeff Wardle said in part, "Several staff and students witnessed this serious accident. Traumatic events like this can be upsetting to hear about, and support is available at school for any students who may need to speak with a member of Student Services or trusted staff member, both tomorrow and in the days ahead.
"Please keep those involved in your thoughts and prayers."
Further information was not immediately available.