Door of German class held by 3 students, but shot threw-- splinters?

Here is a report from the detailed article at Midwest Pundits-- that indicates the gunman shot "through the door" and that "splinters flew everywhere"-- as if it was a wood door. But in a modern university the doors are metal aren't they? And if they're wood, they're solid, and it's hard to imagine a 9mm bullet penetrating a thick wood door so that splinters fly. Also, its reported here that the students used their feet to hold the door-- then later it's their whole body-- and yet none of them were hit?