The 49-year-old was found just after 11:15 a.m., inside Minnie Minoso Academy, which is located in the 3500-block of South Wallace Street in the Bridgeport neighborhood.
He was pronounced dead on the scene. His name has not been released, but Chicago Public Schools said he was a member of the school community.
"Our thoughts are with their family during this difficult time. For privacy reasons, I cannot share further details at this time," a CPS spokesperson said.
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The school was placed on soft lockdown during the "medical crisis," CPS said.
All students are safe, and grief counseling will be provided.
"We are saddened by this tragic loss and will do everything we can to help our community heal," CPS said.
Area One detectives are investigating.