DD8 skipped a full grade against the school's wishes, and we saw evidence of attempted sabotage on the school's part when she began the year at the right grade level. Every problem that came up, they wanted to pin on the grade skip, as a "See, I told you so" exercise.
Given that this is a more dramatic change for your DS, and that the teacher expects it to fail, I would expect to see more of this in your case.
Don't get me started on Everyday Math.
As for adolescent talk, I've actually heard dumber excuses from a teaching professional, who bewilderingly worried about DD's mental state if her classmates grew breasts before her. In your case, would assure the teacher that my child is not being raised in a bubble, and regularly interacts with members of all age groups.
At some level, your DS will be doubting himself, as well, and getting feedback from the teacher that fuels his doubt will make it tougher. He'll need a lot of support at home to power through that doubt.
DD8 has finished her first quarter, and done very well, so it seems those doubts are melting away, and she's being accepted as a 4th grader.
