Impossible.
ILLEGAL aliens cannot apply for citizenship.
you're right, but a legal resident can and a lot of the illegals can change their status. That is what we require in Texas to get in-state tuition. Keep in mind that a lot of illegals are people who entered the country legally but overstayed there visas, work permits, etc.
That's where the catch is, now we know where they are, we know where to find them and in order to stay in school and stay on the program they must cooperate fully with the USCIS. That means they don't skip their court dates, they pay their fines and stay out of trouble. It's not like every person who swam across the Rio Grande get to come to our schools with a discount, there are a lot of strings attached and even though they benefit greatly from the program, in the long run we benefit even more.