I doesn't know, but I does believe that this problem is pretty universal, Ollie. Somewhere around pre-adolescence when we begin sprouting hormones and the drive to fit in and have ambitions, friendship bonds become harder to forge. I believe it is because we grow more complicated and judgmental, but theories abound.

I wish I had advice on how to jump this hurdle, but ? ? ? Instead I will be checking back to see if anybody has advice that might work for me, as well.

Try not to overthink it too much. At 17, you will be making a lifetime of new friends!

Some good places to make friends - at work (can you get a part time job now, or this summer?), volunteering (such as an animal shelter, etc), recreational sports (I played volleyball recreationally through the local parks and community colleges and made a TON of friends by doing that, softball is good too), join a gym, frequent your local coffee place (you may start to see some of the same people there and you can just say "Hi how are you today? What's that book you are reading?").

It takes some practice, but you will get there! :) Start slow by smiling and saying HI to a new person... at school, at the grocery store, someone passing by on the street, etc. To start getting your courage up :)

On the practical side, early in being out on my own I found that going to the same places at the same times/ days was helpful to gaining at least casual acquaintances- if not friends. It does take a bit more effort once the social side of everyone going through mostly the same things at the same time (aka- grammar, middle and high school) ends. But, going to the same classes with people in my major when I was in college helped and after that things like going to the same coffee place every day at the same time, the laundromat too, anywhere where I'd be there for longer than a moment but there was still a defined ending time (my laundry is done, I've finished my coffee) was a good way to at least see the same faces.

On the less practical side, friendships are tough. I was talking about this with a very wise, awesome friend of mine recently. Both of us agreed on a range of things that I'll pass along- that there's a difference between a friend and an acquaintance, that there are people that you talk to about the weather and what a tough test the one on Monday was and so on, and then there's a friend who knows a bunch of random things about you that you wouldn't really tell the person you talk to about the test with. And then, there's the "inner circle" -this is a prized position that certain friends earn by being there during tough times, enduring transitions in life, and navigating changes in each other's time, attention, focus, boundaries etc. those friends are deeply trusted with "where the bodies are buried" level of things. In my adult live, this has almost always been just 3 people (although not always the same 3)
Loosing an acquaintance tends not to bother me, oh, okay, we've moved out of whatever we had in common and gotten on with our lives, I may miss our quick "chats" but, it was never more than a quick chat, I can find new quick chat people. Friends, yeah, I miss them when transitions occur or one of us gets super busy and distracted but, they tend to turn up eventually and I hold in my mind that they aren't in my inner circle and maybe right now, they need theirs and I'm not a part of it and that's okay. For me, it's the "inner circle" people who loosing is gut-wrenching an it's happened a number of times. I lost one of them last year after 17 (yep, as long as you've been alive!) years of friendship. It was extremely hard at first but, with a year to think about it off and on, it's taught me a great deal. I knew that friendship was ending, but, I was making excuses for it/ her/ me, ignoring my gut, trying to alter myself/ my perspective/ the friendship (talk to her less, maybe then we'll have more to say to each other? Yeah, it didn't work but I was trying it was a very long friendship!) We were growing apart, I knew we were growing apart, I knew we had been growing apart for close to a decade. My boundaries were changing again, my focus was changing again, what I required from my friends was changing again- and this time, she- unfortunately, was not shifting to meet my needs and, in hindsight, I know I was stubbornly refusing to meet her's either as what she wanted from me as a friend was not what I was willing to offer her and vice versa- we'd arrived at our parting place, I was going one way, her another and what boundaries we had set and what our needs were didn't sink anymore. It's very sad but, sometimes it helps to look at it that way- to strip the ending of the friendship from all the particulars of it, the particulars of the unmet needs, whatever it was and see it as 'I needed on thing, they didn't meet it" "They needed something I wasn't willing to give" a lot of my friendships, when boiled down, that would be the jist of how they ended- we were going different ways and wanted different things, both of our boundaries shifted, or at least one of ours did, and it just wasn't working anymore.
I don't say this to make you fall into despair. It's happened to me enough to realize that when a friend falls away... I don't think it's ever happened for me without massive growing to follow. And new friends. People who align better with the new emotional place I am in. Sometimes it takes time, sometimes it comes with a period of intense loneliness and grief for the old friend.
For example, the awesome friend I was talking to about all of this a few weeks ago? Well, she made her way to inner circle status right after the 17 year friendship with my other friend ended by rising to the occasion of all the tough things that were going on for me at the time and that I just needed a strong but not overbearing supporter at the moment. She met me where I was at, the more a person grows, the harder these people are to find unfortunately, but the more you grow, the more awesome that inner circle becomes. I wouldn't trade my current 3 for anything- however, I to accept it happens sometimes, often during times of transition for one or both parties and sadly, I have to be aware of that. It doesn't always happen, some friendships navigate transition really well but, it's the most often time unfortunately, at least, in my experience.
I hope something in here was helpful for you. You're not alone by any means, and it IS difficult. I wish you the very best!

Part of the process in moving through the stage at age 17 is learning the ways of people. As each person grows, their tastes, their view, their actions reflect their learning and lifestyle.
People come into our lives sometimes for a reason, sometimes for a season, sometimes for life.
School connection often change as events happen with leaving school. People often get together later on at school reunions. Sometimes, they reconnect there.
Choose what you want most of all in spending time with others. Do the simple things, and enjoy the easy moments of being in same place for a specific reason/purpose.
Best wishes!
You keep writing, and it will help you sort your thoughts.

I can understand how you feel about your friend network collapsing under you. I am old enough to be your grandmother and I feel the same life. This happens in life, because life is like this. Change is something we can always count on. Its not bad, and yet it doesn't feel good sometimes. The way to make a friend is to put effort into things. Say hi to people, smile at them, ask them questions to get a conversation started. Focus on learning about them. There is an old book, How to Win Friends and Influence People. You see, people like us if they think we like them. That is how con artists find their victims (they pretend to like someone and be their friend only to end up using or abusing them). And people love to talk about themselves. Think of this as your project or your goal. Yes, it might feel a bit unnatural but all of us feel odd or weird learning new skills and then suddenly we realize "we have this." Ask people about their tattoos, their favorite music, the movie they went to or the concert, or what they like about some class or teacher. Be natural, be interested in others, smile, give compliments and I would bet you will draw people into your charm and grace.