So, you want to do well on the project, but you also want to maintain your friendship. There are a few possibilities here: You could just say, “No” to your friend, but that would probably make him feel hurt and rejected. You owe it to your friend to explain your concerns rather than just dumping him.
If you think it’s possible that your friend could work well with you, you could say something like “The last time we did a project together, I ended up having to do the whole poster myself the night before. If we work together again, I need you to promise that you’ll do an equal part, and we’ll plan things so we get it all done without having to rush at the end.” To help your friend keep this promise, you’ll need to have “mini-deadlines” to keep you on track.
If you’re sure that working with your friend isn’t a good idea, then you need to say this gently, emphasizing that you like him even though you don’t want to work with him. You could say something like “You’re a great friend, and we always have a good time together, but I think the way we work is too different for us to be partners.” Rejection, even if it involves a project about famous mathematicians, always stings, so if you do this, be sure to invite your friend over as soon as possible, to show you really do like him.
Another option is to expand the group. If you have three or four kids working on the project, and the others are all good workers, you won’t feel overburdened, and your friend might follow the get-it-done style of the other members.
Best wishes in friendship,