The easiest route would probably be a certificate of completion of 1st grade from the old school. Tell them you just got it in the mail or something.
If she has one of these, you can take it to the new school and show it to them. If you have one of these in hand, the new school's only option might be to promote her to second grade (I don't know state law where you are, so...).
If you don't have one of these certificates in hand, call the old school and ask them if it would be reasonable to give her one. Explain the situation and ask what they can do to help.
Val
