Ok to get the social shit outta the way.
1. DO NOT pigeonhole yourself into a particular click. (Yes, even if your a member of a click from elementary school.) Your in highschool now, you don't have to be a jock, or prep, or goth, rocker, hip hopper. Sticking out in a crowd, is not being an individual, especially when hundreds of other kids think their sticking out too. Just be you.
2. Expand on your group of elementary school friends by taking classes they are not. Odds are you'll see your old friends in Science/Math/Geography/English.
3. (I hate to add this one in but...)Unfortunately there are dicks everywhere. Some juniors and seniors see it as the sole mission in HS to make your first year of highschool suck. So don't be a shithead, but don't take anyone shit either.
4. If your an introverted kid, try to break outta that pattern by at least saying hello to people who approach, and be personable and friendly in groups. If your extrovert don't be a attention seeking git. Again just be you, but no extreme's.
5. Lastly, enjoy your time is HS, it goes by very very very fast. Also you will most likely be really nervous, that is ok, so is everyone else.
Academics.
1. Don't get into a comfort zone, so much more is required of you in the next 4 yrs, it will shock you abit. Push yourself, in the following years take elective classes that you think you would not enjoy for sheer learning value.
2. Always take good notes, add enough detail so in the acting of writing your memory will do better to absorb it.
3. For tests and exams study,(especially if you have forewarning) don't cram. Cramming does not work.
4. During the week, get a good night sleep. Feeling refreshed is awesome.
5. Don't freak out and obsess over your mistakes or a failure, listen to your teachers, strive to understand why you messed up and do better next time.
6. If some teachers in some classes offered extra work toward a better grade or rising your grade, take it if you need and sometimes even if you don't.
7. You will probably hear this from all your teachers from now on and it will sound like bullshit, but it is the truth, everyone starts out at an A/100%, its up to you whether you keep that A or not.
Edit: If you are a true procrastinator, you need to break that habit, make a schedule, stick to it. Write down for yourself at home and school, keep it by the class timetable, and one in your locker. In group projects (trust you'll have plenty) don't let the group carry you, they'll hate you for it, and most likely rat you out. When offered your spare period when your a junior do not take it if you have to make up for a dropped or failed class.
AND CONGRATS ON GRADUATING.
