Academic Progress

Mia just received a post card from the woman who was her preschool teacher last year. It said that when preschool starts in 3 weeks, Mia will be in the same class she was in last year. There was no explanation for her parents' benefit - just a reminder about the orientation in 2 weeks. I don't quite know what to make of this. Here are the facts:

• Mia adores the teacher, Miss Diane.
• Mia's 3rd birthday is in November.
• Last year, she was the youngest in her class by a full 6 months. Some children were as much as 11 months older. She is as tall or taller than the other children and generally held her own, but in some situations -- like the singing and dancing performance at the May graduation program -- the difference in age really showed.
• This school isn't rigid about age-grading children. The classes have animal names (like Elegant Elephants and Zippy Zebras) rather than age names ("the 3-year-old class") Also, depending on enrollment, they adjust the number of classes for these loosely-defined age groups from one year to the next.

Here is what is nagging at me. Before school let out, Miss Diane told us Mia would be moving into another classroom in the Fall where she'd be the oldest child in the group. Apparently they were planning two classes for 3-year-olds, one for older and one for younger children. Diane said it would be good for Mia to be the oldest in her class for a change, so she would have opportunities to be a leader rather than always a follower. I thought that was wise. Why am I now questioning this change?
I can ask the question at orientation in 2 weeks. In the meantime, I have to admit I'm wondering whether Mia did something during summer camp (Miss Diane wasn't there, by the way) that led them to question her readiness for promotion. Is it because she wasn't (at that point) showing much interest in toilet training? She doesn't seem to talk much in school - do they realize what her verbal skills are?
And what will be the consequences of holding her back? (There. I said it. My baby is being "held back"!) Will it slow her progress toward reading?
Jeff and I agree that this is a good school and Miss Diane is fantastic, so my head tells me they can be trusted to make the right decision. My heart still wonders. So, I have 2 weeks to craft a tactful way to say: What's the deal here?