If I accept the numbers of that particular study, which are gross estimates according to the document itself, then yes the illegal aliens cost Maryland taxpayers $1.4 billion. That same document also states, buried at the end, that they estimate the illegal aliens pay $200 million in taxes back. (They throw in multiple factors of 2 to make the numbers come out how they want. I think the illegal aliens pay at least $400 million according to their data). They state that yields a net cost of $1.2 billion, but also give their opinion that none of the taxes the illegal aliens pay should count for anything. I don't agree with that opinion.
Paying even $200 million in taxes back is arguably enough. It's an opinion (not necessarily mine). Because any poor person in America, whether a citizen or illegal alien, would fall into the same category of being a "burden to society."
Having said all that, note that I may agree with you in the end. I am not criticizing anyone's tough views on immigration policy as a whole, only the rationale on display regarding this school story. If the illegal aliens can be deported more effectively, by all means go for it. I do however think that if they are not deported, while they are in this country they should be allowed to go to school, especially because they do pay taxes.