The way the law would work is that what was in force at that relevant time would be the law one would be tried under.
So, with Plyler v. Doe, that came 21 years after his birth, so it wouldn’t apply to his status at birth.
For those interested:
http://en.wikipedia.org/wiki/Plyler_v._Doe
Yes it would, just as the illegal aliens that were banned from Tyler, Texas schools prior to Plyler could go to school after Plyler.
The Equal Protection Clause would preclude the state from establishing two classes of citizens or people.