CEDAR RAPIDS — A drawing of a dog, colored in with a brown marker, hung on the third-grade classroom’s door. Above it, a glass panel provided a view inside Mr. Patterson’s room.Eric Werling peeked through it, surveying the Jackson Elementary classroom through a grim perspective.If someone wanted to harm the children inside, what could they see?“There’s bookshelves, bookcases, stuff in the room that we could move around” that would block an intruder’s view, he told the principal.