If it's a mistake, they'll fix it when they find out. I would probably write a letter to the principal or the superintendent as appropriate saying thank you so much for arranging for my DS to be in the 1st/2nd loop classroom. We agree that this is the appropriate placement for him and he is very excited about school. That way they have to say no, it was a mistake, or acquiesce, in which case you have the confirmation in writing. I would not suggest calling to say I'm confused, I thought you said he wouldn't be going into first. But it's not worth losing your spot in Montessori if the school did make a mistake without realizing it and then moves your DS back to K.

ps - isn't this the type of thing your family consultant helps you with?