There seems to be some misunderstanding about the whole thread.
Anyone who falls in the "average" ability band - whether high or low in that band - will be served well with a good teacher in the mainstream classroom. They will be able to make progress at a rate appropriate to them, and enjoy all of the benefits of the being in a social situation with their peers.
A student with exceptional ability will, with a good teacher, be given opportunities to be extended. They may also benefit from extra extension outside the mainstream classroom, but not necessarily.
A student with low ability (note: not low average ability) will need extra support. That might be in the classroom with a teaching assistant, and/or a withdrawal group. Or a variety of other things depending on their particular needs.
Cornsilk: I am a teacher. Consistently rated outstanding when observed. But just in case these observations are skewed, my students make above expected progress. And in the students' own words (yes I do ask for anonymous feedback, like any professional would) - my subject is now their favourite, the lessons are fun, the homework is fun, and they feel confident about reaching their potential and some even feel quite excited about this new found confidence, feeling that it opens doors for them. And I have a wide range of ability in my classrooms - average of 3 National Curriculum levels.
But I didn't even need to ask. I can tell by the way they respond to the lessons.
Please realise the point of this thread: some kids just aren't going to be the top of the class, and that is ok. It doesn't necessarily mean that they need a SEN label. And in some cases, for the parent to insist on this, the subtext for the child could be: you aren't good enough as you are - there must be something wrong with you. That is my point.