There are different levels of not caring. You have some people who only appear like they care just so they can go throughout life unbothered without having to confront the fact that nothing really impacts them at all.
Then there are people who are so adamant about announcing how much they don't care that it's pretty obvious they do, and are as insecure as most humans are.
And then there are people who truly, really, just don't care and shamelessly follow through on that, like the people in this AskReddit thread that called for people's best IDGAF moments and dang did it deliver.
1. Standardized test destruction.
Senior year in high school, we had to take a second standardized test, sort of a beta for a future version they were rolling out. Before the testing week, they gathered my class together and told us that no administrators could check our tests, and that we would not be getting individual grades. Instead, how well we did would determine how the test was handled next year.
Our class as a whole decided to mark C for every question. They were so proud we finished the test 2 days ahead of schedule, we got Thursday and Friday off that week. The next year, we screwed up their figures so bad, they had to do another beta test.
Tldr; don't tell seniors a test will not effect them at all