S.B.
I taught middle school math here in Texas. Except for the pre AP class (gifted and talented), everyone, including special education students were mixed together in the classes. I am not qualified to teach Kindergarten, but speaking from the perspective of teaching older students, it needs to be a mix of both.
While I can appreciate that this teacher doesn't see it as the student's jobs to be teachers, there is a benefit to both parties when higher achieving students work with students who may be having difficulties. Sometimes a student suddenly "just gets it" when a peer explains it. Even if the student said the same thing I said. Sometimes those kids are more willing to ask questions of another kid, rather than the teacher. If you look at Bloom's Taxonomy (basically the learning objectives), teaching a skill requires a student to use the higher levels of learning. So the "teacher" also gains a great deal in the exchange. And if a classroom teacher is rounded enough, there will be times when those who usually struggle will have the opportunity to also teach. (Many of my special ed students were far more visual learners than others, so they often mastered tangrams and such faster).
On the other side there are times when a lesson has to be taught "on level". When those students who excel are allowed to delve more deeply into the lesson. Given a chance to move and little faster and be challenged some too. And students who need that extra guidance need a chance for some hand holding, slowing the lesson down and more repetition. You can't expect everyone to learn at the same pace or in the same manner.
Teaching can be quite the balancing act. And next year when class sizes are larger, it will be even more challenging, but I suppose that's a topic for a different post. :)