neji-aniki: the reason people were surprised was that

1) naruto was giving off an immense amount of chakra....even kakashi and zabuza could sense the demon fox's influence and neither of them have byakugan.....hiashi already knew that naruto had the demon fox 12 years ago, anyway...just like all the other adults

2) hiashi and neji were all surprised because naruto was able to bring out chakra after the 64 hands of hakke was used on him.....im sure if neji figured it out, they wouldve made that clear

as for the naruto failing the exam thing......just as i've suggested before, its a very good chance that they hold graduation exams multiple times a year.....most people dont seem to consider this possibility. Naruto couldn't have started the academy younger than the others cause they show him in their classes prior to the exams