Don't worry about kicking people's ass, because it won't get you anywhere.
I doubt that most people in your class will be able to write honestly about their faith. I say this because I frankly don't think most people ever question the reason they have a faith. It is - especially with young people - something they either inherit from home (as you point out) or something they are experimenting with.
Christians don't have logical reasoning to back up their faith. They have *faith*. That cannot be explained by logic. So I predict that a lot of those papers will have a lot of mumbo jumbo about Christianity which has nothing at all to do with why each of the people on your class are religious. Not because they are stupid, but because they cannot explain it better.
So, to the point. Why are you an atheist? Think about it. Try to write down some reasons. I'll try to help you get started with some points, then you can come back and I am sure others will help you out too.
There are also many sources on the web which might help you get ideas for what you can write about. For example
http://www.infidels.org/news/atheism/
1. Atheism is not a faith. Why?
2. Question the need to have a logical reasoning for a person's belief
3. What is it like to live without a faith? Why does it work for you?
4. What could be the reason you don't have a faith?
Your teacher has an obvious agenda here. How can you give a logical reason for not having a faith if you never had one? It is like explaining the logic behind why you don't have a thousand dollar bill when in fact you never had one.