Someday I'd like to tell this to my son:
Be nice. Be kind. Be upstanding. Be honest and fair.
Show kindness especially to the people who you feel are below you. There will be kids who are a little bit odd, socially awkward perhaps, unkempt or poorly dressed, sad or serious, shy or clumsy. The other kids will tease them and it will seem to you that you should tease them too, just to be safe. Don't do it. Be nice to them. Maybe they'll never be your close friends, but if you're cruel to them, you will forever be their enemy. If you're kind, you may become their ally. If you think you don't want to be their ally, just wait. You may change your mind.
If a classmate says something careless or hurtful, stand tall. You don't have to smile; you don't even have to answer. But don't let yourself be crushed, either. Everyone says mean things sometimes. That kid probably said something thoughtless or mean to some other kid yesterday. If he's trying to push your buttons, don't let your buttons be pushed. If he's being offensive by accident, forgive him and let it go. You have more power by ignoring him than by reacting to him. If you ignore him, he's helpless against you.
Share. Especially if you have food to spare. Nobody can truly dislike a person who offers him food. Especially cupcakes. One of my closest childhood friendships started with cupcakes.
Kindness won't solve every problem. It won't guarantee that you'll be the most popular kid in class, with everyone clamoring to be your friend. It won't fend off every bully. But it works a lot better than a lot of other things you can try.
Remember, the nice guy always wins. You may not win immediately, but just wait for your reward and it will come. Even if your reward is just better self-respect, that alone is worth it. You know you did the right thing, and that's worth it. All those kids being mean to each other - they're going to regret it someday.
There will be times when you can't be kind, you forget to be kind, or being kind is not the best thing. There may be times when letting the other person have their way will actually hurt you, and you decide to stand up for what you want instead. That's okay. In time, you'll learn when it's important to oppose someone. But even when you do oppose someone, do it respectfully and honorably. Don't kick them when they're down. Your adversaries are people too, and if you're kind to them even while you compete with them, they'll remember.
Most important of all, be kind to weird people. Having some weird people among your friends is valuable. You'll find out how valuable they are as soon as you make some weird friends. Just trust me on this.