My DS is in the same situation as yours. Was in a montessori school skipped kindergarten and is now the youngest in his 2nd grade class (January 13) so very similar. He skipped kindergarten in the montessori school BUT they would not move him up until he could write. One summer I spent everyday practicing with him. That is the one thing I had to push with him. But I think it had to do with a great teacher. She knew if he could not write no matter how smart he would not succeed in skipping a grade. I was grateful to her and it forced us to really work on writing, which of course he hated.