I was in a connecting classroom, we shared learning opportunities, and I never saw her exhibit any sexist or harsh behavior. In fact, boys who were in her prior classes came back to visit her often. Yet, she couldn't shake this distinction. I wish it were so. I had never learned Montessori methods, never taught in a Montessori school.
Parents pass impressions of teachers along like dog-eared treasure maps, and so do children.
This is all to say, that your son needs to know that what he has heard about his teacher may or may not be true. Encourage him to wait and draw his own conclusions. Because teachers and children have different personalities and learning styles, a poor match for one student may be an exceptional match for another. Making up one's own mind about other people is a skill he will need now and for the rest of his life. To help your child feel more relaxed, ask him to tell you specifically why he is dreading this teacher.