The video posted online shows what appears to be a security guard pushing a female student down the stairs and then later striking her after she gets up and moves toward him.
The alleged incident on Tuesday has been the talk of Dunbar School according to this student who didn't want to be identified.
"Some of these students have smart mouths, so generally I thought it would happen sooner than later," said the unnamed student.
CPS says the security guard has been suspended with pay as police investigate.
And a spokesperson in a statement said: "The safety of our students is our top priority. CPS takes matters of employee misconduct very seriously, especially when it involves an employee whose job it is to ensure the safety of students. Once CPS was made aware of this incident, immediate action was taken to remove this individual from his position while the investigation continues."
This Dunbar student says fights are common at the school and guards often have to intervene.
"Mostly they try to break up any fight, but I've never seen it get that far," said the unnamed Dunbar student.
Unclear is what precipitated the alleged actions of the guard, though some students say the girl may have pushed him first.
The student's mother has now hired an attorney, and in a statement he said: "We are shocked by the unjustifiable assault committed by the CPS employee?The video and the egregious actions of the CPS employee speak for themselves."
The student has not returned to school since the incident on Tuesday.