It's a crazy thing: that moment when you realize that your high school days are coming to an end. Looking back at the last few years, I laugh at all the mistakes I made, all the weird things I wore because I thought they were stylish, all the times I freaked out over things which, in retrospect, are quite insignificant. I wouldn't change much of my high school experience, I don't think, even though there were some days where things went horribly wrong. But I do wish I'd known a few things when I stepped into those bustling hallways on the first day of freshman year. Yes, there are definitely a few words of advice that probably could have saved me lots of time, energy and tears.

1. First impressions matter.
I used to think first impressions were overrated. High school taught me that they aren't. This applies to teachers and friends. Be authentic (as in, don't try to be someone you're not), but try to start off on the right foot. It's natural for people to remember their first impression of you more than the following ones. Be mindful of your bad habits. Play up your strengths. You'll be glad you did in the long-run.

2. It turns out, most people don't even notice what you're wearing because they're too concerned about their own outfit.
There's nothing wrong with wanting to look good on the first day of school, or every day after that for the rest of the year. But don't stand in the mirror second-guessing every item of clothing you try on. Choose your outfit based on what allows you to feel confident and comfortable, instead of what other people will think. People will likely catch on to sincere confidence before they take note of what shirt or shoes you're wearing. The truth is, for the most part, other people are caught up in what they themselves are wearing and what people will think of it, just like you are.

3. Take risks.
Try out for teams. Audition for productions. Run for student government.
When I showed up at high school, I was fairly overwhelmed by all the new opportunities I had access to -- sports teams, theatre productions, music groups and bands, clubs, student government, and so many more. With so much to do, there's really no time to sit there questioning whether or not you're good enough to make it; just go for it. You'll never know if you don't try. And believe me, things that feel like failure or rejection in the short term end up being some of the most meaningful and valuable experiences, even though they're tough. When I look back, I realize that I don't regret the times I tried out and didn't get it; I only regret the times that I let my fear get the best of me and didn't try at all.

4. Upperclassmen aren't that scary.
Those juniors and seniors strutting down the halls aren't really all that scary. They just want you to think they are. They're not going to hurt you. They don't rule the whole school. They aren't on a mission to embarrass you. In fact, for the most part, they don't pay too much mind to the younger kids on campus. Now that I'm an upperclassmen, I can say that I like friendly underclassmen who aren't afraid to strike up conversation. BUT, don't forget about a key thing called seniority. They paid their dues as underclassmen. They got bossed around too. Don't mouth off. You're not too much younger, but you're still younger.

5. There is a difference between "cool people" and people who are cool.
You'd think that the people who end up in the ever-so elite "cool" or "popular" group, are just that: cool. As it turns out, that's not always necessarily the case. That's not to say that there aren't popular kids who are cool, interesting, and fun to be around; there absolutely are. But the more time I've spent around the "cool kids," the more I see that there's nothing particularly cool about them that separates them from everyone else. And there are just as many kids who reject that label of "cool" and consciously opt out of that social scene who still have really cool skills, personal stories, hobbies, and are definitely worth getting to know. Bottom line, one's position on the social hierarchy does not define who they are as a person, or their level of "coolness."

6. One grade is just that: one grade.
I had a teacher once who used to give a speech virtually every time she handed back an essay or exam. "This is just one grade," she'd say "in one class, for one semester, for one year, at one school, during one phase of your life." I'd sort of just laugh nervously, hoping she wasn't trying to soften the blow of a bad grade. But over time, I realized she was right. No single grade is going to determine your fate. One grade is not worth your tears.

7. Some teachers are actually really cool.
There are a lot of teachers who are really cool, and funny, and they almost make you forget that they're grading all your papers and giving you exams every other week. In all seriousness, when I started getting to know my teachers, I found quite a few who were really easy to talk to, had really good advice, and were very admirable overall. Teachers will notice when you take time to get to know them. They can be a really valuable resources, and not just in an academic sense.

8. Knowing how to collaborate with peers is important.
If you talked to students who worked on group projects with me during Freshman year, most of them would probably say that I was overbearing, controlling, and not easy to work with. Frankly, I was. I wanted everything to be done right. I worked with plenty of peers who didn't put enough effort in, thereby leaving more work to me, and that was difficult too. Collaborative projects are inevitable, starting now and forevermore. It's really important to communicate with whoever you're working with. Everyone works at their own pace and in a slightly different way, and working in a group can be insanely frustrating. But it's a very important skill to have.

9. Don't write off a bad teacher immediately.
There's nothing worse than going into a new class and discovering that you really don't like your teacher. But don't write them off too quickly. Sometimes it gets better over the course of the semester or year. Some teachers are harsh at first because they want to establish themselves as authoritative figure. Some teachers take a few weeks or months to adjust to your individual class. There's no harm in being optimistic and giving them some time and space to change your mind.

10. You need sleep.
You are not immune to exhaustion. You do need to sleep, perhaps now more than ever. If you don't get enough sleep, you'll suffer consequences. It's as simple as that. (P.S. Don't go around bragging about how little sleep you get. That's not a competition you want to win.)

11. BALANCE. BALANCE. BALANCE.
High school is the first time in your life when you really have to start juggling lots of obligations, without a lot of help. It's also the first time you realize how key time management is. Striking the right balance between academics, extra-curriculars, friends, family, and your own well-being is the hardest thing I tackled in high school. Harder than any test, any fight with a friend. It's an ongoing process. But it's immeasurably important. If you can't figure out how to strike the right balance, you'll burn out pretty quickly and won't be able to do any of it very well.

07/18/2014 03:27 pm ET | Updated Sep 17, 2014

Carly Steyer 18-year-old from San Francisco, CA; HuffPost Young Voices intern