I found this very enlightening:
DIFFICULT MORAL QUESTIONS
Question 171: May undocumented workers be employed in violation of the law?
As the recently appointed interim principal of a Catholic school, I have inherited some undocumented immigrant employees who have been paid off the books for several years. My predecessors apparently had no problem with this arrangement.
On the one hand, I realize that employing undocumented immigrants involves technical violations of the law, though many generally law-abiding people seem to be doing it. On the other hand, the present situation seems mutually advantageous to both the workers and the school, since they probably could not get better jobs elsewhere and the school probably could not hire others to do the work so cheaply. Thus, terminating the arrangement would involve rather serious hardships for both parties to it.
I am wondering what I ought to do about this situation, and how grave my obligation is to do it.
Analysis:
This question calls for the explanation and application of norms regarding just wages and conformity to laws. Undocumented workers are willing to work for less not because their work is less valuable than that of other workers but because they need the work more. Employing them to save money unjustly deprives them of the money that is saved. Paying them off the books both violates presumably just laws and wrongly deprives society of tax revenue—things contrary to the common good—and deprives them of the benefit of having paid the taxes that were due on their earnings. Even if immigration laws are unjustly restrictive, employers who violate them are unfair to fellow citizens.
Employing undocumented workers also is unjust to citizens and legal immigrants who could do the jobs. A Catholic school that engages in this practice gives bad example to its students, impedes its witness, and risks bad consequences for the Church. Therefore, the questioner should take steps to end the employment of the undocumented workers in a way fair to them.
The reply could be along the following lines:
Although you do not explicitly say the school’s undocumented employees emigrated due to economic hardship in their home nation or nations, I shall assume that was the case. Employing such workers is unjust for several reasons…