Read This Book!!!
Setting Limits in the Classroom (Revised) by Robert J. MacKenzie, Ed. D.
I had problems like these in a lot of the classrooms that I sub in--just going by the teacher's posted rules never seem to work. I implement the suggestions in this book and they really work!
I make clear expectations and consequences before beginning a lesson; I take note of those not paying attention. Then, you check in with student and ask them to please pay attention because instruction will not be repeated. Then, if they still keep talking, initiate the consequence. A logical consequence for chit-chatting during instruction is not having the instruction repeated and not letting another student help them. Have the student make up the time during recess or free time so you can re-explain instructions ON THEIR TIME. Another option would be to write the instruction down or give the instructions before the bell rings so they have to do the assignment for homework.
Of course, if a student is struggling because they don't understand is different, but for not paying attention, I stick to my guns on this. Let them sweat it out!
I really like this book because it shifts responsibility for behavior to the student without making them feel less of a person or stressing the teacher out.