Tell them you're taking a graduate-level course and:
1. There is no teacher.
2. You never have class.
3. Your classmates are some of the top analytical minds in the world.
4. There is only one exam.
5. 60% of the class will fail.
6. The class is only offered once a year.
7. Repeat that very same process 10 times.
8. Oh, and you're competing against the top 40% from the previous class.
Most people get that. And then they shut up.