Public schools educating illegal immigrants is not only not illegal, it’s legally required by the U.S. Supreme Court.
Bond buyers are fully capable of making informed judgments about illegals in the schools. The poverty (measured by free- and discounted lunch) and racial and ethnic demographics of every school in every school district are published by the U.S. Department of Education. You can easily compare those to the racial demographics published by the U.S. Census Bureau to understand demographic trends in the district.
The point is to start the ball rolling-—one never knows what’s going to crawl out from under that rock.