I'm happy you do know that discrimination is being used as a term of art. However, for this hypo, the fact that the state is requiring proof of lawful residency in this country IS in fact the discrimation. Immigration status enjoys a higher level of judicial scrutiny given its mention in the constitution. To those that feel immigration should be addressed by the states should direct their words and their efforts at congress, and ask they give them that power, rather than hope in vain that the court is going to overturn Plyler v. Doe.
What irony. Circumvention of the Constitution is best accomplished when one finds one's self using the artificially created analysis doctrines the Court uses to legislate. The doctrines are, nonetheless, real.
You seem to be arguing not about discrimination, though, but rather that the Federal preemption doctrine precludes states from getting into determining immigration status. I agree they cannot do that. I remain unconvinced that requiring proof of residency rises to the level of discrimination, or that it even triggers the lowest level of judicial scrutiny.
As for the suggestion to write one's Congressman, that will not help with Plyler. As you should know, Plyler cannot be undone by an act of Congress. It will take a Constitutional Amendment or, for Congress to overturn a prior holding.
That's never a safe bet. Indeed, the principle of stare decisis makes it hard to fix bad law. Having said that, I think the point of the author is that the Court now has different votes than it once had, and it has advantage of hindsight.
Right now -- today -- there is no other feasible way to stop the illegal aliens from using public schools than to overturn Plyler.