>>But if teachers proctor students from the same school, then there is pressure to cheat coming from the principal and superintendent (as in Atlanta).
Well it depends. A lot of the really crappy inner-city schools have high turnover so you have a lot of non-tenured teachers who will do anything not to be fired through attrition (proof of being warned thrice), which is something that the admin can do very quickly through observation/duty schedules/class covering/parental complaints/etc. When a teacher becomes tenured (3-5 years) then the admin has to show about a billion steps taken to get the teacher on the right track.
Something tells me that these were newbie teachers right out of school that were willing to do anything to keep their jobs.