I would say that both sentences should use “adónde” because the question is really “to where”, and the “a” adds the “to” part. The adónde can be written as one word or as two words, “a dónde”. I think the course asking you for “Dónde vas tú” is either an error, or is giving you an intentionally sloppy/casual/incomplete sentence along the lines of "where ya goin’?
I’m not an expert - I think this is a great question, and I hope you get more replies. Which course is this in?