Our beloved Montessori school never asked this until just before our K enrollment - they operate strictly on a first-come first-serve basis, and probably also know that circumstances change and parents change their minds.
One important point, though: despite the possible attitude problem of forcing parents to disclose their plans, the Kindergarten year of the preschool program is CRUCIAL.
There is significant attrition in the K class for 3 reasons:
1. Cost compared to public school.
2. Parents wanting to send ALL their children to the same school as soon as poss to avoid complicated commutes (if the older ones already attend elementary).
3. Parents not understanding the true value of Montessori K.
I can speak for number 3:
Academically, Kindergartners in Montessori learn so much more than in the public school - the math particularly is light years removed in rigor and depth. Ex: my son did the thousand chain, which besides learning to count to 1000 is also a springboard for cubing numbers. All his classmates could read fluently by the end of the year (my son has a speech delay and was the only one who could not reach that particular goal). The critical thinking involved is much more rigorous than in public school and thus is a much better preparation for academics than public K.
Socially, the Montessori mixed-aged approach really comes into its own for the older kids in the class. After 2 years of looking up to the older children as guides and teachers, they finally get to experience being looked up to and teach! It's incredibly affirming and confidence-boosting for them, and quite precious, as later on in elementary school they'll revert to being the younger set. And as we all know, teaching something makes you understand it on a more profound level, so the Kindergartners develop a rock-solid foundation of knowledge.
Some parents are concerned that their child will not integrate well if they come in public 1st grade instead of K, which makes me smile, because so many families in this area move around a lot, so that there is no guarantee a public K student will see a lot of familiar faces in 1st, and anyway children that age are very flexible and make friends easily.
I moved to three different countries before age 8, and my young preschoolers and elementary-aged children have moved to Europe and back - it has not impacted their ability to make friends or their social life at all.
I have no idea if answering "no" would get you turned away (I hope not!), but you could say yes if any of my arguments have intrigued you. You are also allowed to change your mind.
Most of all, maybe you should look into a different, more welcoming Montessori school
