Why are some scenes so freakishly dark ?

I agree I could not see anything, the sound was bad on mine too

lighting wouldn't make or break a production budget. Plenty of lower-budget films are shot and lit just fine. You need a DP who knows what they're doing, and in some cases, this person seemed to have a great eye for composition.

The day scenes are wonderfully lit (the scene specifically where she's teaching the children in their rows of desks in the hallway), but the darker scenes are overkill. They made an obvious choice to go "practical" with these scenes and use minimal lighting. If anything, you overlight these scenes a little, and then darken them in post.