I know that they really can't teach more academics in full-day K).
Full-day K at this school just adds in the fun stuff like another recess, lunch, and specials like art, music, PE, computer lab. But yes I can see that maybe the more advanced K'ers may have ended up in full-day classes. It actually costs $$$ to "upgrade" to a full day, but our choice to do half-day was based more on our desire to have more time with DD and to allow her to participate in her various activities without having such a packed schedule.