Cabinet member for schools and lifelong learning Ian Jones has apologised on behalf of the council, which has now withdrawn the flyer.
In a written statement sent to PRWeek, he said: "I was upset and dismayed by this image and have apologised for the offence it has caused. This has included a direct apology to both sets of parents."
The picture in the flyer - now the subject of a council investigation - was not spotted when it was proofed for errors, he added.
The leaflet was produced by local community arts group The Public, and emailed to Sandwell schools, some of which emailed it to parents before it was recalled.
The image shows a young girl with a water pistol held at point blank range to another girl's head. It appeared alongside a calendar promoting cultural diversity, prompting complaints from parents and teachers that the image sent out the message that guns are fun.
The West Midlands council has stressed its continued support for campaigns against gun crime.
The authority covers the area where two teenage girls, Latisha Shakespeare and Charlene Ellis, were shot and killed as part of a gang war on New Year's Day in 2003.
The council pointed out that its comms team did not approve the leaflet.