But Naruto entered Kiba's class as "the new kid", implying that he was transfered from some other class, since at that time, he was already supposed to know the Henge perfectly...

my guess is that they have a few exams each year (in the first ep, Iruka says that even those who already passed the exam should do the henge test becuse of naruto), and based on that, they rank each student and form teams at the end of the year...