My DD6, also doing 3rd grade spelling, got 15/16 on her first test and 13/16 on the most recent one. Her three mistakes were simple ones like leaving out a letter or writing a "z" backwards. Her teacher is talking about giving her harder words.

Now that I ponder the idea, I think that kids who can spell like this would benefit from doing something different than classical spelling.

I would vote for vocabulary and writing exercises, with spelling corrections made in text.

Val