Police said an 18-year-old suffered non life-threatening injuries during the incident near the branch on St Lawrence Road, Darlaston.
It happened at around 3pm on March 20 with six teenagers, aged between 14 and 17, being arrested in the aftermath.
They were arrested on suspicion of assault with intent to cause serious harm and have been released on police bail while enquiries continue.
Sergeant Sarah Clinton, investigation manager at Bloxwich Force CID, said: “This was a nasty attack. Our investigations are moving at a fast pace and we have made six arrests.
“The attack happened in a busy area and we believe people would have seen the incident and may have information. Witnesses or people with information should contact us on live chat via our website or by calling 101 quoting incident number crime number 20/178815/23.”
Alternatively call Crimestoppers anonymously on 0800 555111.