No one takes a hit off a joint expecting to get hooked on crack or other drugs, it's just marijuana right? But that's the first step on the steep trek to rock bottom. Especially since it's very common now for marijuana to be laced with other drugs. People like to mix cocaine or PCP in with the marijuana, and most of the time you won't realize it's in there until you've already smoked it.
You may think that your friends doing drugs seem fine, but drugs lead to serious problems - with school, parents, society, the law, work and other friends who don't use. The problems start to build up, and continue to grow. People get out of touch with themselves emotionally, and wind up being numb most of the time, not able to cope, and then instead of using drugs to feel good anymore, people use them to feel less bad.
And not everyone does them, though it can start to seem like that when your group of friends starts. When you start using the excuse that "everyone" does something, then the cliche comes up "If all your friends jumped off a bridge would you do it?" You are an individual, you are responsible for your own actions, your own outcomes. I hate that cliche that I quoted above, but it leads to this answer: "If all my friends were to jump off a bridge, I wouldn't jump with them, but instead go catch them at the bottom."
And that's what you should be thinking about. You don't have to stop being friends with them, but don't put yourself in positions where people are going to be trying to get you to jump off bridges either.
Tell them you're just not into the drug scene. You may have to ease off the friendship for awhile until they come to their senses, find some new friends, or be strong enough to realize that you don't want to be a part of the drug thing, and don't let anyone tear you from your values when you are with them.
That is where self-esteem comes from. Sticking to what you believe in, and not letting anyone talk you into betraying yourself. Never betray yourself. When you are around them keep firmly in mind what you want out of your life, and that jail cells and crack alleys are not the roads that takes you there.
Every time they offer you drugs, just look them straight in the eye and say "No thank you, I'm not into that." Say it again, and again until they stop asking.
You seem like an intelligent person, and I'm sure you'll find the strength to stay true to yourself.