It's the weirdest by far.
When you go to a graduation a year before your own, it's not as bad. Sure, it's sad to see your older friends leaving, but you know that you still have time until it's your time to walk the stage. At your own graduation, everything is a blur. There are pictures and speeches galore, and you're really just going through the motions. Going back is when it gets trippy.
You sit there and listen to the same speeches and see the motions, but it's with the knowledge that you'll never go through that again. I was in a class of 76, so, by the time we were graduating, we knew each other pretty well. Whether we were best friends or not, we knew it would be weird not seeing the same faces we had seen for seven years. Emotions were high because you realized your life wouldn't be the same old routine anymore.
Going back for the first graduation after your own is emotional, but for a different reason. You see all your old teachers, both good and bad, in one place. You get a hard-core sense of deja vu watching everything unfold. You can't help but compare the two ceremonies.
And, the most emotional part of it all, was watching as your younger friends, your honorary baby siblings, receive accolades and walk that stage just as you did the previous year.
It's just hard to wrap your mind around. Graduation was an event that you looked forward to for many years. You imagined what it would be like as you walked in that cap and gown, but then it was over in a flash. Watching it all unfold, you realize what it was really like to be there.
Even if you don't know the class beneath you all that well, I would say you should still go support them. No one will know better than you what it feels like to leave the community of your high school. The action of going to that first graduation as an alum will act as a benefit to yourself just as much as it will the graduating class.