Groups.

You are who you hang out with.
It is a simple fact. Even if you do not have the same beliefs or morals with the people you hang out with, in people's eyes you will be associated with them and all their ideals. And if you are hanging with people who do not have very strict guidelines for themselves to follow, your lines may begin to appear a little fuzzier the more time you spend with them.
You should not care what people think. I would say that is a point-blank fact but there is some grey area around it. Let me revise: You should not care what irrelevant people think of you, however, your friends that want the best for you, your parents, adults that respect you, should not fall in that category. Believe it or not the reputation you have with the people who you should care about is what is going to benefit you in your life. Your teachers are the ones writing recommendation letters, you friends are the ones that are sticking up for you (or at least should be), and your parents love you, and to love you is to care about you and see you make the best version of yourself present in all situations.
If you hang out with people that disregard authority, do not mind people's feelings, or do not hold any of the standards you hold from yourself you should not let them be influencing you.
I am in high school, I understand how difficult this can be. It is hard enough to find a group of people that all like you let alone are actually all decent people. Once you find a group that will accept you it is often really tempting to conform because no matter what kind of people they are, it is better to be included and invited than it is to be alone. People in high school also seek out the best offers, so if you are trying to find that good group of people and you find one person, the makings of a good group, that one person might jump at the opportunity to hang out with another group because apparently a party is more appealing than just a night with you and them.
Do not let the ever-changing friend groups of high school define you. And just start laughing and move on when the group of friends who you thought were your friends turn out to be awful and go seek out the individuals who will always care for you.
JSL