I had my 50th High School Reunion last year. I had planned to attend but wasn't able to. It would have been the first reunion I would have gone to. I wasn't missed.
Like everything else, memory depends on your personal experience. I would have like to have attended but I also would have not have known anyone, that is except Cindy Williams, who also didn't attend. I was a loner in high school. Not anti-social, just not social. I have a couple of classmates that live here in Vegas but other than making comments on Facebook posts, we don't talk.
I know of people who have kept in touch over the years but most people drift off on their own paths, never to be seen again. But, unless you're from a small town, where everyone knows everyone else, remembering or not remembering someone probably depends on whether you are a piece of a favorite memory or not.
Being as young as you are, you have plenty of time to make yourself an indelible memory on the hearts and minds of those you know now. Depending on what you do, you may even stir up memories in those who you once knew.