There is so much in your senior year of high school that matters to you then. It doesn't matter if later in life, you don't care... for a lot of kids, those moments are what matters to you completely right now.
Prom, honors night, graduation... not for every kid, but for a lot, those are really important events. And once they are gone, they aren't coming back. You can't send kids off to college this fall, and then NEXT spring have them back for graduation. They miss out permanently.
We have to be ok with those kids being sad, mad, devastated and angry. We can expect them to behave appropriately while having feelings (e.g. no hitting), but we have to let them have their feelings and not try to moderate them for them, excuse them, or dismiss them with "none of it really matters," or "you aren't going to see any of those people in the future anyway."
Do you remember being 17 or 18? IT MATTERED.