I would probably recommend something like "I don't consider myself Christian anymore," and leave it at that. This makes it entirely about yourself and your relationship with the religion, and not a comment on anything that anyone else does or doesn't believe. It will probably make people less defensive, and the non-committal nature of the statement indicates that you're not looking for a fight or a long conversation about how you came to where you are.
As Paul suggested, try to change the subject if you don't want to go into it, but if you're are asked what you do consider yourself, or whether that means you no longer believe in God, you can always just say "that's personal, and I'd rather not discuss it right now."
Some people are going to push, and some of your former fellow members of the Campus Christian Fellowship, or whatever, may be very distressed and try to persuade you to come back. If you choose to discuss it with them, just be aware that you're unlikely to convince them of anything, and they're probably just going to become more distressed and more vehement if you try to tell them why you became an atheist. So, if you don't want to get drawn into long conversations about religion, the best thing to do is to make simple statements and then move on. Remain friendly and calm, and try to talk about other interests you still share with them. If they won't change the subject, just say you've got to get to class, or do some homework, and get out of there.