What rights does a veterinary student have dealing with forced unpaid labor in the hospital setting?

We are veterinary students in our senior year. In addition to our assigned service for our clinical rotations, we are on call for ICU shifts where, if called in, we assist technicians perform medical treatments, walking dogs, or cleaning. The school will not hire an additional tech but instead take advantage of the unpaid students. There are other paid positions for students, but this is not one of them. We were recently told that the student on call from Sunday morning from 1am-8am is now required to come in to work regardless if help is needed. This to me seems like forced free labor since we are taking the place of what should be a paid tech. We are writing a petition and need legal advice on what makes this illegal. They say this is learning experience for us, but cleaning and walking dogs is not something we need further learning on. We did enough of that in order to get where we are today. Thank you.