Since this was summer work, I don't think the penalty on you would be much because F1 students are not required to maintain full time enrollment during summer breaks so if they are threatening to do that you should tell them to go ahead since you're going back to school and are complying with the law. They cannot do anything. Homeland is not going to deport you for this. Tell them if they report you to the Homeland you will also report them to the state employment office and IRS because they have also broken the law. Also if this was a fast food place I am assuming it is a franchise. You can even start by telling them that you will report them to the franchise office first before even going to the State or the IRS.