Explanation fits description.

Academy City was treated like a city made up of hundreds of schools, but it was more accurate to consider it like a city-sized boarding school. It was large enough to cover a third of Tokyo, but was surrounded by something akin to the Great Wall of China; though not as strict as a prison, it was still not a place one could simply wander into.

...Or so it had seemed. In reality, three satellites launched for experiments by a technical college were constantly monitoring the city. Every individual going in or out of the city was completely scanned and if any suspicious person for whom the gate records did not match, either Anti-Skill or Judgment from different schools would immediately intervene.