let's say naruto starts in the academy at 8. he fails three times (1 fail per year, assuming test is given every year). naruto is 11.
kiba enters, same age as naruto, after naruto's 3rd fail. both are 11, they both graduate. they are now 12.
or something like that, i don't see how it's not possible for naruto to have entered before kids like kiba or shika. the age might be different but as long as naruto enters 3 years before kiba does, it should work, no?