The 11-year-old girl was assaulted in New Road, Tipton, at around 3.45pm on Tuesday, West Midlands Police said.
The attacker fled the scene and remains at large.
A man previously arrested on suspicion of being involved in the assault has now been released with no further action.
A spokesman for West Midlands Police said: "We are investigating after an 11-year-old girl was sexually assaulted in New Road, Tipton, at around 3.45pm on Tuesday, March 30.
"The attacker fled and we are continuing our enquiries to identify him.
"A man was arrested on suspicion of being involved in the assault but following further investigations he is not believed to be responsible. He has since been released with no further action.
"We understand how traumatic this was for the schoolgirl and have offered her specialist support."
Anyone with information is urged to call West Midlands Police on 101, quoting incident number 20SW/90889G/21.
Alternatively, witnesses can call Crimestoppers anonymously on 0800 555111.