A girl who attends Roots International Academy says she was sexually assaulted in the school's locker room after school.
Police think the attacker may be another middle school student.
The girl told police it happened January 26, but did not report until a week later.
"We can't really speculate on why the victim did not report it immediately. There are all sorts of emotions and different influences on these cases that affect why these things evolve the way that they do. All we can say is that once we did receive the report we took it with the utmost seriousness and began the investigation," said Troy Flint from the Oakland Unified School District.
A letter detailing the investigation was sent to families on Friday.
In addition, the school has put a district police officer on campus to increase security.