I'll be the one hold out here as someone who does not like split age classrooms, at least for the kids in the older grade. We had some of them growing up and my brother, who was placed in a split one year as part of the younger grade group, found that he wasn't socially or emotionally mature enough to be with the older kids. He was teased and laughed at for still believing in Santa, for example.
The school my dds attended when they were younger offered a split with parental option of requesting that placement or requesting not to have that placement when dd13 was a 2nd grader. I asked that she not be placed in the 1/2 split and was glad I did so as the year went on. The parents of other 2nd graders who needed academic challenge felt that their kids made little academic progress that year.
I think that the one room school house example is a totally different thing on the other hand. There you have 20 kids from K-12 so most of them are on completely different pages. It is more like a group homeschooling co-op with a facilitator in that situation b/c the teacher isn't likely to be standing in front of the class and teaching anything to a group. S/he's more likely to be devising individual lesson plans for every student and moving throughout the classroom working with small groups or individuals.
I don't know that I'd pull a child from a school she's attended up until now due to this one thing unless you have not been happy up until now, but I would be keeping a very close eye on the situation, her academic progress, and I'd be somewhat irritated by the principal's attitude of 'I can do whatever I want, so take it or leave it.'