I'm having flash backs to law school. It's a difficult world to negotiate. I don't remember that part of my graduate school experience very fondly. Of course, I had 200 people in my class, so at least I had a bigger pool from which to choose my friends.

In your case, I suppose you could do one of several things. You could simply not study with them and limit your relationship to just being friends with them. Next time they ask you to join a study group, say that you think you process the material better on your own.

Depending on the depth of our friendship, you could instead try to express your frustration--tactfully--without it blowing up into a big fight. That's what I might do. For instance, the next time a study group session goes off track, say to them "guys, I feel like you're all depending on me a little too much to teach you the material...." If you really feel like A is the biggest problem within the group, perhaps you could just say that to her privately.

But, obviously, you have to choose your words carefully here. There are better and worse ways to assert yourself. I'd rather you took that tact that simply bottling up all your frustration and then blowing up at them. That won't help your cause either.

To me, this situation is an opportunity for you to practice learning how to calmly and appropriately assert yourself. I think women in particular aren't very practiced at doing that, so we either bottle everything or become really passive/aggressive in our dealings with others. Neither approach is healthy, nor does it do much to build good personal and professional relationships. I wish I'd learned a long time ago to (a) be more honest and direct with people and (b) not try to fix everyone's feelings. Remember that there is a middle ground between avoiding people entirely and taking whatever it is they dish out at you. The quality people in my life are the ones who can deal with me laying appropriate boundaries to take care of my own needs without taking it so personally.

The only other thing I'd add is that you come off as a little, um, egotistical in your post. Perhaps you really are the smartest person in the room. Good for you if that's the case, but what I've learned over the years in my professional and personal relationships, is that a little humility goes a long way with people.