Do what you want to do man, but I am simply saying that there are provisions for foreign students to work. From the top of my head I know that certain visas like the J1 student exchange visa and the F1 visa have provisions that allow students to work. The M1 visa may have some provisions as well but I am not sure. All these could be visas that your students may have and diiferent rules may apply. People may talk of "under the table" but at the end of the day there are either laws that were violated or not. I suggest you vist uscis.gov and see the rules for visas for clarity before making such huge accusations.Then again, as I said before, do what you want but be informed that just because someone is on a student visa does not neccesarily mean that they are working illegally.