Well, we all entered the dungeon without any problem. We entered the same portal but we didn't come out at the same place. It was established that we would be transported randomly but the only guarantee was that we would end up in the Outer Region of the dungeon.
The outer region is the safest place in a dungeon. As we go towards the core the more dangerous it becomes. So the academy has set up this dungeon so that we would all end up in the outer region, rather than in the inner or core regions where the most powerful monsters live.
Normally it would be difficult for that to make this happen in a dungeon. When you enter the dungeon, you would be randomly transported to anywhere in the dungeon or there would be a single point of entrance. It is either this or that, it can't be controlled.
The reason why the academy could do this was because the dungeon we entered was an Artificial Dungeon.