I don't mean to brag, but I'm smart. Very smart. I have a really high IQ, I'm a hard-worker, school work comes naturally to me. I'm in a special program starting junior year where I take college course so I don't have to take my senior year of high school or college. I've always been in advanced classes + gifted and talented. But I'm terrified of leaving my friends. The only older friends I have will already be graduating earlier than me anyway. I'm scared of being alone at only 17. I'm scared.
I have two brothers and they both ended up in gifted and talented programs, skipping grades, etc (my brothers may be smarter than me but I'm a way bigger deal on tumblr*, so we're basically even) and it did hurt them both socially, I think. I don't think it necessarily had to be that way, but it's not like most people in high school think being super smart is super cool, you are separated from most of your peers when you enter a special program, and you end up as the youngest person in your college courses. It's not a great recipe for a robust social life.
I think the first thing to consider is that there is always a transition from high school to college/the workforce/doing nothing but smoking weed in your parent's basement for a few years - whatever people end up doing. The final teenage years through the early twenties (and beyond) is a period when people change a great deal and really start to figure out who they are. Most people don't keep all of their friends from high school. I still talk to some of my old friends, some are acquaintances on facebook, and some of the people I was friends with all through my childhood, through high school...if I don't see them again that's fine by me. It just sort of happens. People change in different ways so we didn't even fall out. We just ended up doing different stuff.
What makes this all doable is that life isn't so much about abrupt starts and stops as it is about transitions. I doubt you'll be "alone at only 17," or that any period in which you feel more isolated means you're doomed for life. You simply meet new friends. College is frankly amazing for that. I was a crazy dedicated student in college and I found friends who were also very dedicated. Your people are out there.
Skipping grades and taking college courses early, you do have to face being the youngest person in the class. This seems to play out one of two ways. Usually (and this was the case for my brothers), the young go-getter is PAINFULLY shy and ends up socially isolating themselves. The other option seems to be being completely full of yourself and making everyone hate you. If you can not do that but also not be totally shy, you'll do just fine. And honestly, what do you have to lose? They'll know you're smart. If they're smart, they won't say NO HIGH SCHOOLERS IN THE STUDY GROUP or whatever. And if they aren't crazy about you, it isn't like high school. You aren't stuck with these losers forever.
I know it's scary, but you can always ask, "what's the worst that can happen?" You can try to make new friends in your classes and maybe they won't want that. Okay, but if you sit apart and just think about how these people aren't your old friends, you're guaranteed not to make new friends. Try not to do things that guarantee the worst outcome, you know?
You'll be just fine. One of my brothers skipped a grade early in school and it took him a long time to fit in socially. He figured it out, though, and he's doing great. There was a kid from my younger brother's class who skipped ahead into most of my high school classes (which was weird, admittedly). He was super shy and super weird, but he has a lovely family, did all sorts of incredible stuff in college, and is A-OK.
Life does not end at 17, even if the transition from high school is made a little harder.
Also super smart but never felt the need to show off like his stupid brothers,
*My brothers aren't on tumblr.