Iris had gained permission from the academy, not only for herself but also for all the people who agreed to help John get a fruit a life, to go out of the academic ground for nearly two weeks. The receptionist did ask her why and where she was going with the others, but she gave the receptionist a false reply. She wasn't willing to break the truth of the Tomb Of Roselia to the staff of the military academy as they would most likely tail them.
Not a lot of people knew about the tomb of Rosalia and its location, especially not the ordinary members of the military and its affiliates. And as a member of the Twilight Temple, Iris has no intention of letting this information be made public. Since otherwise, it will aggravate the greed of many people. Some people with no chance of surviving an encounter with monsters in the demonic. And some that would do anything to get the treasures of Roselia.