Hi, Sarah, I'm sorry to hear you're struggling. You aren't the only one who have a problem keeping friends and you aren't the only ones who have "friends" that you just say hi to, it's been like that for me for a long time. Too long that I don't even want to think about it.
You're probably a freshman in high school now, and you should really focus more on your grades. I understand wanting friends to hang out with and chat with, believe me. But there's also schoolwork and other responsibilities. In a few years, you'll be able to drive and go places on your own, places where you can meet people and make new friends.
I'm always here if you ever want to talk. I know online friendships don't even come close to real life friendships (like, we can't go to Starbucks or the mall obviously) but it's just as good, if not different. And believe me, my online friends are a hell lot better than the ones in real life.