If I could go back go freshman year, I'd do the following:
Do well in school, get good grades. High school is bullshit, you know that, your teachers know that, your parents know that, but you have to do it anyway. Why not make the most of it. Homework and studying can be great tools to practice discipline, and trust me, the shit you deal with in school is nothing compared to adult day to day.
Pick two hobbies, one to keep you in shape, and one that could make you some money on the side. Seeing as you applied to a conservatory I'm assuming you're either a dancer or musician, if it's the former, keep that up! Don't worry about trying to dance professionally or anything like that. Most people who turn their high school passions into adult careers end up despising it. Joining a school sport is also a great idea, you stay in shape, learn discipline and bond with your schoolmates, win win win.
Use the internet as a tool, not as a time vacuum. You may need varying levels of discipline for this, but a good start would be to unsub from anything that's not at least tangentially beneficial to your future. You have all the knowledge of all men and women who've ever lived at your fingertips and you're using it to giggle at gifs? No. Learn a language (duolingo, news in slow), get ahead of your classmates (khanacademy), or just fucking go wikipedia hopping and learn random facts. Anything is better than a time sink. Budget your computer time.
Volunteer or get a job. This doesn't need to be a full time thing, but I never worked in high school, and let me tell you how shitty it was to stare down my first full day of work at the ripe age of 19. Really shitty. Seriously. Get a handle on this stuff while you're young. Lots of the jobs available to young women are actually pretty tough when you're just starting, I personally just cannot work in food service because I am not a manic ocd extrovert, but most jobs for you through school and early adulthood will be in this industry, so you may as well get a hang of it.
Make three plans, short term/midterm/longterm or alternatively 90/365/1,826. And you can have more than three. Make social plans, career plans, travel plans, hobby plans. Planning and acting is the key to long term success, practice a few short term plans to see how you do before you commit to something serious, you do not want to make a habit of giving up.
Take your opportunities as they come. Take them by the scruff of the neck and don't let go. If you get the chance to travel, take it, or to go see a show, or go on a date, or go to lunch with your mom. Say yes to things and never ever give away great opportunities because you're tired or in a bad mood or feeling anxious. Those are temporary emotions, shrug them off and live the life you want to tell your grandkids about.
Good luck, I know none of this is as easy to do as it is to type, but your youth is the greatest tool you have, please don't waste it looking at photos of cats.