Nightshade spawns at night time instead of evening, the "evening" time in the academy is like before 0:00 a.m, I guess? So Nightshade spawns in the night as its name suggests.
The Witch academy is still under development, like you said walking out of the academy buildings at evening makes the game jam, boss fight bad ending is not complete yet, and after boss fight, seems like there's no more further content at present.
And leaving the academy cause a bug that After leaving the academy, the academy time mechanism is still displayed at the top of the screen. When returning to the academy, the "check in" process start again, but there's nothing you can do because the quest is long over.