D.B.
This is the teacher's job. It's called classroom management. There is absolutely NOTHING you can do from home that will make a child stop talking at 10 AM or 2 PM. If the teacher doesn't want to resolve it, set up a conference with the school psychologist or guidance counselor or principal (with the teacher as well).
But this is either kindergarten or first grade, right? And it's September. It's early, and kids are getting into the swing of things still.
Did the teacher give you any more info than you have given us here? Not much to go on re other factors like special needs, medications, what's prompting in, or what has been tried.
Any teacher that expects you to fix this from home needs more professional development. That doesn't mean you can't be involved, and you should learn what is being tried so you can reinforce it from home, but otherwise this is up to the school to handle.