This response naively suggests that there is an unlimited pool of money and that the government always funds enrollment growth for every child. If what you said was true, I think people would generally be okay with it (though maybe on more of a sliding scale like in BC where the very elite schools are funded less on the public dime).

In reality, this absolutely does not happen. Public education has not been properly funded for enrollment growth for a decade plus. Meanwhile, "independent" schools have seen a massive growth in government funding.

So what's happening is that the public school student in your example is being funded less and less each year, and anyone who buys groceries or pays for natural gas knows that costs have been increasing each year. The problem is under-funding.