Say no calmly if necessary. Saying, "No, thanks" can be enough for others to realize there's no need for further argument, but other times people may want more than just an answer. They might want an explanation from which they can formulate another argument. It's important when dealing with these kinds of situations that we remain calm and collected. Remember: even though people are being rude right now doesn't mean they won't suddenly become nice tomorrow...or later today after school ends.