Allen did not even know what happened after he left the cafeteria. But even if he gets to know about that he will just not care about them. If you are thinking that he became an arrogant person then you are wrong. From the first, he just only cares about two people. Fast his teacher and then second the principal of The White Tiger. Other than that he did not care about those students.
Now come back to the main thing,
After coming back to his room as quickly as he can he quickly brings out his system screen and taps the dungeon option. Just by typing that option, he vanishes from his place.