For one thing, we project the merit of parents (or lack thereof) onto their children. We do this in how we allow massive intergenerational transfers of wealth. But also, as you point out, the schools a child can attend are very dependent on who their parents are.

In Chris Hayes' excellent book, Twilight of the Elites [1], he rejects meritocracy. Not only does he reject it in practice, for reasons like those I presented, but he also rejects it even in theory. He suggests that the idea of showering people with resources that show promise is an affront to the concept that every human being has fundamental value and that all should be invested in.

Liberals, broadly speaking, do love schools. But I don't think that's for lack of concern about other issues of social justice! Perhaps you mean economic liberals (e.g. classical liberals and neoliberals)?

[1] http://www.amazon.com/Twilight-Elites-America-After-Meritocr...