Well, 'We are across the library' doesn't really make sense in English. At least not in the context of this sentence. It would be 'We are across from the library'.
That said, I feel Duo might be a little off on this one as I think 'across from' would be more accurate for 'enfrente de'. For in front of I'd use 'frente de'. Though research shows there are many conflicting opinions on this so I'm not %100 sure about it. A second opinion would be great.
Edit For 'We are across the Library' I'd say: Estamos al otro lado de la biblioteca.