For what are hopefully obvious reasons, I will not allow a student to conduct a study that I see as unethical. Before any study is submitted to them, however, it will need to be approved by me. If I don't fee that subjects' rights are being adequately protected, or that the nature of the study places people at-risk, it will not be forwarded. If a study cannot be done properly in the time you have available, it defeats the goal of trying to graduate. Students are often afraid of replicating previous research.
Replication is a necessary part of science.
We'd be fools to rely on the results of one study to determine the legitimacy of a theory or idea. Or I may urge you toward more of a conceptual replication where the idea is tested in another way. Make sure you start by looking at existing research and ask yourself how you can add to it. Groups larger than two students will need to be justified based on the complexity or time-consuming nature of the project. In any case, it is expected that each student is providing a unique contribution to the project and that work on all parts of the study will involve all students assigned to that project.