My DD3 is only about to begin at a private Montessori that goes through the elementary years so I don't have direct experience with your situation but I have been thinking ahead. I was reassured by our school that they keep materials in the primary classroom (ages 3-6) up to second grade level and they happily (and regularly) pull materials from the upper elementary grades when necessary to meet the needs of the kids in that class. When I toured there was a K student who was reading at a 5th grade level and they insisted they were able to meet her needs in that setting. Have you been given any assurances that they will focus on your daughter's individual needs regardless of grade level? Are they open to reevaluating at a later date?