Maybe the wall is covered with whiteboard-paint? But then again they may be using permanent markers so I guess you're right.
Apparently "the girl writes on the wall" is not correct but it should be since she does that now so the verb "write" has to be written with an "s" at the end.
Note that it's multiple girls doing the writing: pigerne. A single girl would translate to "Pigen skriver på væggen."
So am I right assuming that the consonant "v" is spelled like "w" in english unless it is at the beginning of a word- like here, and being so, it is spelled as "v" in english??
It's never a distinct [w] sound. Danish pronounciation is really lazy, mumbly, so when there's a 'v' written, you say it like a [v] but with your mouth more open than usually.