Immunizations And Schools

So I'm registering my son for kindergarten and I made a doctor's appointment with my son's pediatrician. My son is up to date on all of his shots!! Signed by his doctor! I went the school to register my son for kindergarten and then the office clerk said that he was missing shots? Even my doctor thinks it's weird that the school is trying to stop him from registering for kindergarten. Okay I waited until he was 3 to get all of his shots. Saying he's missing his Hib! But he's not because I had proof with a shot record.