To me, this (among SO MANY OTHERS) is an INCENTIVE for citizens from other countries (ie: illegal aliens) to want to come to America illegally--to give their children an education they probably wouldn't receive in their home countries.
Remove INCENTIVES, remove MOTIVATIONS to break America's laws.
Pretty easy to understand, actually.
My opinion is that it would be foolish to bar kids from school before we took other steps aimed at keeping people from crossing the border illegally. People try to come here for many reasons aside from the schools, and they will bring their kids regardless of whether the schools will let them in or not. And I don't want legions of kids running around outside of school.
That also seems pretty easy to understand.