You're 15. That's about the age when people consider you a "young adult," so welcome to the club! It's an interesting journey, being a young adult. There's the fun parts, the extra responsibility, the trust, and a whole bunch of other stuff. But there's also the sad parts, like actually being held responsible for your work, or taking part in harder classes. It can be difficult, but we can make it fun.
It is simultaneously weird and amazing that I can call my baby sister a "sophomore in high school." That is insane. It scares me because I remember being a sophomore like it was yesterday, but I am so excited for all of the fun things ahead of you, like driving and real social events. It will be a fun ride, I can promise you that, but that doesn't mean that there won't be a few hard times.
Fifteen is when times get tough; Taylor Swift wrote a song about that age, so it must be true! Friends will come and go, and I am always here to make sure that you are never alone. I have been where you are now, and I probably paved the path for you a little bit. I'm sure that I've made mistakes, and that we have both (hopefully) learned from them. That's the beauty of being the youngest: you have at least one siblings to always watch and let guide you. If I have guided you or helped you, I've done my job. If I haven't, that is okay too, because that means that you get to make your own mistakes, and maybe you will get to teach me something I never knew before.
Classes will get harder, and teachers don't baby you anymore. Work hard, because I never want to hear you say you "regret you worked harder" or "regret not putting as much effort in." There are no do-overs in life, and you don't get the chance to change something you regret. So push yourself, and do more than what you though you could do. Also, do what is best for you. You're probably all confused after hearing college talk after college talk between older siblings and parents. I'm sure guidance counselors intimidate you, and you're already dreading the SAT. Don't. It isn't worth your worry yet. You'll have your time to be an adult, and you will have to make big decisions eventually, but not just yet. Loosen up sometimes, and enjoy still being a "young" adult, rather than an adult.
You are probably fine on your own. Actually, I know you are more than fine on your own -- you will thrive and grow and experience wonderful things as a young adult. It will be more fun than not. I am so excited to watch you do amazing things, and live vicariously through your high school career. I can't wait to see what the future holds for you, I am also beyond blessed to have a best friend and baby sister like you.