You are probably right.
I think there can be a lot of lying with statistics (on both sides). This might be one of them (i.e. under counting a negative effect). Of course the adults could be legal or the children could be legal ("anchor babies"), either way they would not be counted as illegal aliens using public schools. Of course the taxpayer feels the effects regardless of legality.
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I was also NYPD, I know a housefull of illegals when I see one. Although I am certain that you are right about a few of the babies.