Be alpha and don't be anyone's bitch.
take this advice not in the sense of "look cooler than everyone else", but rather in the "don't join a group of idiots first thing".
unless you know there are some friends of yours coming to the same high school as you that you know super well, i would suggest staying pretty quiet, like all freshman are for the very first day. see who your teachers are, learn their teaching methods, get your books, hopefully see some students you can befriend later, but don't attract attention to yourself straight away.
make the teachers like you, but don't be a suck up. if you get A's on every test, you're more likely to impress them rather than raise their expectations of you. if you get B's on your assignments but always remember to turn in everything assigned to you, that could potentially impress them as well.
first day, keep your backpack light. a few things to write with, a few notebooks to write in, but otherwise keep it empty, you'll be bringing quite a few textbooks home.
by the end of the week, make sure you know at least one person in every one of your periods. this is important because groups start quickly, and having zero friends, in the worst case scenario, could make you the loner of that class for an entire year.
my freshman year, which ill be leaving at the end of this year, was pretty horrific. i was the loner in one of my classes, had pretty much only one friend in every other period, and had very little people i knew come to the same school as me. however, i excelled in pretty much all classes, became well known by all my teachers, and the friends i made are also friends with my friends, so we're all one big group of nerds together.
