What you call "most and least gifted" is in my opinion better described as "most and least privileged". And coeducation of students of different levels of privilege just seems like a good thing, nothing that needs to be dealt with.
I agree that schools should not differentiate based on families' status and wealth. Schools should treat poor and rich children the same.
However, schools should never treat stupid and intelligent children the same. Not everyone is cut out to be a rocket surgeon. But your argument means that we should hold back all future rocket surgeons and bring them to a lower level of privilege, i.e. dumb them down. This is neither in the interest of the children (stupid as well as intelligent ones) nor is it in the interest of society. We do need rocket surgeons...
I agree that this can be a difficult balance to strike. But I think it's also important to keep the door open for children who mature a bit later. Stamping someone as "not gifted" by excluding them from a "gifted" group sure seems like it would cause problems, especially for younger children. Of course it's a difficult practical problem to solve; to give every child challenges on their current level.
Also, while there are surely variations in intelligence that are "from birth", I do not know how that compares to all the variations caused by different educational privileges; having parents that have a lot of time to read for/with the child etc. It sort of comes down to "equality vs. equity" I suppose - and that is not a simple question.
Schools do not treat stupid and intelligent children the same, teachers - with few exceptions - do. Not because they are evil, but because to be able to cater to the different levels of intelligence and the variation in interests that you will naturally find in a random sample of children they require the time and opportunities to do so.
As long as we treat teachers as the least important workers, without acknowledging the critical work they do [0], have classes that have more pupils than a teacher can follow individually in a meaningful way and consider education "an expense" that needs reduction, rather than "an investment" we will not have a proper education system.
On the flip side: an uneducated population is easier to sway, so there is not a lot of pressure on the political establishment to change things.
The things we should invest in are more teachers, smaller and more separated classes and better teachers' education. Only the last one correlates (somewhat) with teachers' salaries.
Asked in a different way: if we paid (all) teachers 3x as we do right now, would we observe significant long-term improvements in students?
This is odd - I am not using code blocks, I am just typing in the text box to reply. I thought it looked like code as a way of showing my comments. I really don't know why it happens, I will look and see if it's somewhere in the settings.Personal experience: I had a great math teacher in mid school; he gathered the best students from the 5 classes in that year in one class and he set the baseline quite high. For the last 2 years half of our class was taking the prizes at all the inter-school competitions in the city and a few of us from the nationals. Most of that class went to the best high schools in the city and most are today great doctors (medical) or scientists. Imagine that did not happen in mid-school and everybody would be an average Joe - that is a loss for humanity, not just for us.
Try to read Ender's Game (the book, the movie is terrible). This is very tough, but if you need great people you can help by creating the conditions to grow them. How many Einsteins wasted their lives achieving nothing and how earlier curing diseases and improving technologies was possible if that waste did not happen?
The compromise is to waste the resources that we have in the name of equality; we cannot make everyone smart, we only can make them all stupid, so let's do equality.
There is nothing saying you can't have good classes for both high and low performers, but with schools that have limited resources it often works out to a poor medium where no one really thrives (hence the caveat of 'in my experience' in my comment).