You need to convince your teacher that Minecraft would work just as well (if not better) than Sims, and give your reasons.
It would be best to appoint a spokesperson if possible, and agree for the rest of you to be quiet unless the teacher asks them a direct question.
Think of your reasons beforehand, try to make valid points and if possible, try to think of a reason why Minecraft would be better, as part of the assignment, than Sims would be.
Don't just say "well, Minecraft is better than Sims," it doesn't matter which game you prefer playing, he/she needs to know that it works for the assignment. If it truly does, there should be no reason why it can't be used.
To be honest, I would think that if you can base an assignment around a game that you enjoy playing, and know a lot about, the whole assignment will be better - both from the teachers point of view, and also from what you as a group will learn from it.
Good luck, let me know how you get on!