Even ignoring the highly questionable aspects of the pledge which you carefully omitted from your quote, nationalism is just the grotesquely overgrown brother of tribalism, itself a badly flawed concept. At least within a tribe, it's hard to keep secrets or conceal abuses of power. It still promotes an unthinking herd behavior, a sense of "us vs. them, and clearly they're worse than us or they'd be part of us". At the national level, it fuels wars and xenophobia. It is the tools of propagandists and of those who would re-write history and get away with it (as you yourself noted, with regard to Jackson).
I find it disgusting that a nation which arose out of a rebellion against government mistreatment tries to brainwash its children into giving their allegiance to anything so inherently flawed as a human government. Would you have supported colonial children in the 1770s being required to stand up every day in school, and swear allegiance to the Union Jack, and the monarchy for which it stands? Do you think it's cool that there are probably kids right now swearing their allegiance to the People's Republic of [Korea|China|the Congo|whatever] and the glorious freedom and representation that their government bestows upon them?
Liberty and justice for all? Give me a break! Pure propaganda, and you don't even need to be *that* smart or well-educated to see it for the lie it is; you just need to start from the assumption that the American Way is *not* The One True Way, and look up some facts. Facts like per-capita prison population, or the breakdown of said population relative to the populace at large. Facts like the mere existence of places like Gitmo. Facts like the government's treatment of Snowden, and their hasty effort to scrub from their websites, etc. all mention of the Obama administration's moral and righteous promises to protect and support whistleblowers. Or how about the states where gays, or transgender people, are forced to live as second-class citizens (and, in a handful of very backward parts of the country, criminals)? The very concept that there exists "one nation, under God, indivisible, with liberty and justice for all" is a tremendous lie. Teaching our children that such a thing not only exists, but that they live in it; forcing them to chant those lines every weekday of their young lives to the point that they absorb it before they're even old enough to know that sometimes the things you're taught are wrong? That is beyond the pale. It is despicable and deplorable.
Now, actually pledging liberty and justice, that's not so awful. It should still be taught as a *concept* and not as a mantra, but pledging to protect liberty and promote justice is a noble and virtuous thing to say. Too bad that's nowhere in the pledge of allegiance as it stands today, though. No, we were told to pledge allegiance to a flag and a nation, not a concept. We didn't even pledge to uphold the constitution, the way so many civil servants are required to do.