In high school, you find your people. You find the people you hang out with all the time. You find the people you get ready and take pictures with before you go to prom and homecoming. You say you're going to be "best friends forever," and stay in contact once you graduate.
That doesn't always happen.
I was one of those people that thought I would stay friends with my buddies after I graduated, but that did not happen. Not even close.
We talked a lot the summer after I graduated, but that was about it. I left for school, and that was kind of the end of our friendship. I used to ask my friends constantly to come visit (I'm only 3 hours away, so I'm not too terribly far), and none of them came. My roommates would have some of their friends come up for homecoming, Halloween, etc., and none of mine would come.
It bothered me.
I quickly found that I didn't have true friends. I didn't have people who wanted to make an effort to be friends. They didn't really want to be a part of my life, and that took a while for me to accept. I didn't want to admit that my friends weren't really my friends.
Since coming to college, I found new people. I found people who make an effort to see me and talk to me. I found people who want me in their life, and it isn't just a one-way street.
While losing friends can be hard, it can also be a good thing. You learn who actually cares about you, and you see other's true colors which is good for you in the long run.
So yes, friends in high school are good, and it isn't the end of the world if you lose them after you graduate. You'll find better ones in the future.