The superintendent released a video overnight explaining the short notice to parents.
The threat was reportedly made to Cleveland Middle School on Monday.
Superintendent Stephen W. McCanless didn't give specifics of the ongoing investigation, but said the threat was serious.
"I want you to understand that when a student is suspended, all the way through December of this school year, and we implement a new safety policy so quickly, that is a message to you and the community that this threat needs to be taken seriously," McCanless said.
Cleveland ISD's clear backpack rule takes effect on all campuses starting Friday.
McCanless said if parents are struggling to find a clear backpack in time, they should talk to their campus administration.
"We do what we need to do here to take care of our most precious asset, which is your children," McCanless said.