I understand what you are saying and no doubt that some, but not all, of the difference is due to the factors you have cited. I don't disagree. As you know most private schools don't have the teachers for the special ed kids. But to think that they number any where near a population to affect the overall score in a meaningful way is a smoke screen. My nephew had a learning disability and the closest public elementary school nearest my house (they were living with me at the time) didn't support these kids. So they also had to move him to another school. Even the close school did not have the test scores to match up.
The school bus was the same one that he would take to the public school if he had attended it; all funded with my tax dollars. And yes, the monies expended from fundraisers and the like are in the annual budget.
What I vehemently object to is the claim that money will fix the problem because the schools are underfunded. Only people who are altruistic would say that in most cases. And there still is no disputing the enormous sums of state money going to the inner city schools and the correlation to the high test scores versus no state money to St. James as you mentioned and the higher scores.
Are there underfunded schools in a few communities? Absolutely. Find them, make the case, and get the funds from the state or the local taxpayers. If I am working a low paying job and my kid's school needs more tax money, I vote yes and see if I can get a part time job or reduce expenses to make up for the slight increase in taxes.
Are all the schools seeking increases in excess of the rate of inflation worthy and needy? No way. Just more pigs at the trough seeking to take more money out of the efficient private sector and increase the public sector bureaucracy. Socialism didn't work in the USSR and it won't work here. Europe is about to collapse from all their "feel good" spending that the "mean" conservative people didn't want to fund. There are trade offs. The US is no longer the best place to invest your money. It has slipped from 1st to something like 11th I heard recently. That is not the right direction for a capitalist society that wants to make things better for our kids and grandkids than we had them.
I truly do agree with dissenters that we MUST provide an education for all the kids in this country to ensure we remain a strong country. We MUST. I understand that.
Please forgive me for the long winded response. I just can't stand it any more....