The increased use of machine learning (ML) in society raises questions of how ethical dilemmas inherent in computational artefacts can be made understandable and explorable for students. To investigate this, we developed a card-based design workshop that allows students to reflect on ethical dilemmas by designing their own ML applications. The workshop was developed in an iterative process engaging four high school classrooms with students aged 16-20. We found that scaffolding students in designing meaningful ML systems served to qualify their ethical reflections. Further students' design processes allowed them to engage with the ethical dilemmas and to tie these to the properties of the technology and to their design decisions. We suggest seeing technology-close discussions about ethics as a goal in design processes, and prototyping as a means to ground these discussions in students' own design decisions, and we contribute a workshop format and design artefacts that allow for this.