Dear W.,
I have read the other responses.
My first thought when I read your request was that instead of getting upset or angry or defensive, (which is hard to do sometimes, I know), you maybe should ask the teacher why she asked you that question in the first place. Not in an accusatory way...but maybe she had a reason. How would she even know or pick up on what your daughter's night time habits are? I'm not defending her by any means, but why would she ask that? Is your daughter reluctant to use the potty at preschool and asks for a diaper?
Talk to her again about it. She might not have meant it in the mean way that some people are taking it. Maybe your daughter told her she still wears diapers. You won't know unless you have another talk with the teacher.
From personal experience, I know kids who weren't allowed at pre-school because their parents just slapped a diaper on their kid. I know a 10 year old whose mother insisted on a pull up every night. Neither of those kids ever used a diaper or pull up at my house or anywhere else, but they let loose at home. Out of habit. That is my belief.
I know that all kids are different. I have a girl and a boy 10 years apart. I never bought a pull-up in their lives. I think that when they have a diaper on at night, they know that they don't have to worry about that urge to go to the bathroom because the diaper takes care of it. Some, not all, parents, would rather have a diaper or pull up than worry about having to wash bedding. Cleaning pee sheets is no fun, but I think kids potty train far better if they can learn to respond to their bodily urges as opposed to just sleeping through them.
The other thing I want to say really quickly is that my daughter started kindergarten at 4 years old. She was ready in every way, shape, and form. The preschool teacher said it would be a disservice to her to keep her behind.
If she had still been in diapers, even only at night, that might have made a difference in what she was ready for. I don't know. My son was easier to train than my daughter as well. There wasn't a diaper in sight when he was 3.
You will do what's best for your child, I have no doubt.
On one hand, it's no one's business how long you let your daughter wear a diaper at home. But, I would take a deep breath and ask the teacher why she brought it up then go from there.
Best of wishes!