It was very rare that a student was expelled from Nova academy. Even the weakest that had fallen to the lowest classes had some power to stay in education. Even worse, Doc had stated that the two Cain family members would be unable to return to the city at all. They would be seen as fools that disrespected the very safety of their city and their family.
Some large families would have argued this. However, when it comes to a dungeon break and imminent danger, things were different. Causing more damage during a dungeon break was ground for execution let alone expulsion or exile. Not many had the authority to do these things. Doc was not necessarily someone that could flat out exile someone.
drop those golden tickets!