I think it takes at least a couple years after High School for most "kids" to realize this.

It doesn't make your HS friends bad people, or bad friends (although some of them probably were), and it doesn't make the "forced friendship" entirely meaningless, but HS friendships are mostly hollow, and mostly don't last.

The reality is that you're a 14 year old kid when you get sent to HS, you're only 18 when you graduate, and most of your relationships within that school are going to be built around the fact that you're all there.

When you go out into the world and spend some time being an adult, you realize that you had some shared experiences, maybe some common opinions, and you may have tons of great memories of the time you spent with them. But your friendships were built around a common like of certain TV shows, certain albums, a mutual dislike of school, etc...

Those friendships aren't organic, and you're going to either "out grow" most of the friends you had in HS, or realize most of them weren't great friends to begin with.

I'm 37 now, and the kids who were the most teary-eyed at my graduation are generally now adults who don't even remember the names of most of their HS friends. I certainly don't.