Secondly, you seem to imply that no improvement in the education level has happened because of the normalization of high-school education. I disagree with this also. I have no numbers to back this up, but by looking at the world around us, at all the inventions and the added complexity and our increasing ability to handle complex systems, I think we have become smarter.
Thirdly, maybe social mobility was hijacked by some rhetoric or became a symbol for this or that group. But at its core, social mobility means allowing people to rise to their potential and not stifling them. How can we be "sacrificing our schools on the alter of social mobility" when social mobility is supposed to be _more_ meritocratic, not less?