Depends on the people. I've been in groups where I would usually consider others to be people who are just along for the ride, and I've been in groups where I did all the work. I've been in groups where I told someone that their idea was stupid and that we were going to do it my way because I was the top student out of the bunch, and I've been in groups where I didn't lift a finger or contribute an idea at all.
I work better alone, and prefer to work alone. Like a lot of others, I'm a perfectionist. Especially when it comes to projects for school. I like having a say in everything, and I don't like it when people ignore what I have to say about something or how it should be done. Most of the time, I think my idea on how to do something is better than everyone else's, and there have been times when I split up from the group and worked completely on my own and did my own project, which to this day I still consider that somewhat badass, especially since I told them that I did so because of creative differences! The teachers weren't very happy about that, since the purpose of group assignments is to see how you work in a group, but I don't regret it.
Besides, I think most teachers don't put the effort into picking the groups. They either make an effort to keep you away from your friends, and stick you with people who are in a different social circle with you (usually one that tends to stereotypically rival yours [face it, we're all stereotyped and placed in a social circle in school]), or they let you to choose your group, and you end up choosing the people you're familiar with: your friends. They should be more observant of the people and what they have to offer to a group, and choose that way, rather then strictly deciding the groups based on who you may or may not talk to.