I had a lot more homework than my peers and was expected to act more mature. Sorry, but we were all the same age as other kids - we didn't deserve a higher workload (as kids and teens) and we should have been expected to act our age.
It was pretty common to make fun of others for not keeping up well enough (struggling not allowed) or for appearing too smart (Not me, but a family member).
Some school systems completely separated gifted kids from 'regular' students. By high school, it was obvious that this created some issues communicating with a broader range of folks.
There is more than one way to make sure gifted kids get challenged - you don't necessarily need a special class for gifted kids.
And you'll need to provide proof for the last one. It is true that they do teach so that the test scores are good - and since funding and jobs are tied to that testing, other things are going to go down. This isn't really making content matching the lowest denominator, though.