It happened at Kilmer Elementary School in the Rogers Park neighborhood's 6700-block of North Greenview Avenue.
The school's principal sent a letter to families, saying some people in the school community consumed an "unknown substance" in the form of "gummy edibles."
Chicago Fire Department officials said they transported three people to St Francis Hospital after they ate something they should not have.
They are all expected to be OK.
Further information was not immediately available.
Principal's full letter to families: Safety is always my top priority, which is why I am writing to notify you of an incident that occurred during today's school day. This afternoon, we were made aware that some members of our school community had ingested an unknown substance in the form of gummy edibles. Out of an abundance of caution, notifications were made to ensure these individuals received medical attention. Everyone is safe. The parents of the impacted community members and the CPS Office of Safety and Security have been notified, and while I cannot provide further details for privacy reasons, I can share that we will be handling this matter in accordance with CPS policies.
We believe the impacted community members may have ingested the unknown substance in the form of "edibles," which are food or drinks that are made with cannabis or other prohibited substances. Prohibited substances of any kind are not allowed at our school, and we encourage you to take this opportunity to have a conversation with your child about the health risks associated with consuming edibles and to remind them not to accept food if they do not know where it came from.
Thank you for your support and partnership as we continue striving to create a healthy and positive learning environment at our school. If you have any questions or concerns, please feel free to contact me directly.