You are using incorrect cases for pronouns, as in "Du bist ebenso gut wie sie". "sie" should be "she" or "they", not "her" or "them". I'm getting marked wrong for being right.
@Darien: Can you please give the full English sentence along with the German one? It's impossible to answer your question if we don't know what the original sentence was...
If I understand what you're saying (you are being marked wrong for writing "You are just as good as she" instead of "...as her"), you are correct. "Is" is implied in the sentence, i.e. "You are just as good as she (is)" - the "she" is in the nominative case, and your translation is grammatically correct.