I’m 17 too, I’ve taught myself some techniques throughout the years. The first is breathing, I take a short breath before I’m about to say something and in between words. It’s helped a lot for me. Another technique I’ve used is identifying where the sound comes from e.g. the roof of the mouth, the lips, the throat, the teeth. When you identify where it comes from you can relax that area or muscle to make it easier to say. Don’t really know if any of those would make any difference to you. I’ve also had the same issue of trying to make friends at a new school, it’s hard but you have to put yourself out there.