You basically said it yourself: you don't WANT to be mean to people. That's not a bad thing. But you also don't want people walking all over you, and that's human and normal. Rather than "being mean", I think you want to practice "being assertive." Not aggressive, but assertive. There's a big difference.
Don't try to come across as a "big ol bully"; if it's not who you are, you'll likely only get laughed at. If people treat you badly, disrespect you, or hurt your feelings... it can be difficult, but merely bring yourself to a place where you can speak up for yourself. Something like, "You must know that I do NOT appreciate your saying those things about me. If you continue, it says far more about you than it does about me, and none of it is positive. If you keep it up, I'll keep you on 'permanent ignore' where you belong." Later, you might try asking, "do you kiss your mama with that mouth?" (You'll need to be feeling uncharacteristically secure about yourself before you do that!)
Until you can do any of the above (the first time is the most difficult, believe me!), just give them disapproving glances and then ignore them. But when you're ready, telling them what you think of their behavior is NOT mean; it's assertive, and it's perfectly acceptable.
But if you can avoid a huge conflict, that's never bad. Just don't let them walk all over you. (Basically, I just told you how I handled the same type of situation, years ago.) Can you speak to a school counselor or a teacher you respect, and get his or her opinion?