We only have prom and fancy graduations for seniors in high-school. We also have "White nights" where students party all night in their school yards, freshmen give you flowers and stuff, you have a diploma ceremony, prom, then a holiday with your classmates away from your city/village.

I'm gonna miss mine due to obvious reasons, sadly, but it makes a lot more sense because graduating high-school means you're about to move onto a completely different chapter in your life, especially since a lot of people study abroad these days, and the younger students rest just have small celebrations that last a few hours with their classmates, maybe a gift from their parents if they made a deal such as "If you pass with [grade] or more we'll get you [something]"