I'm going to be cold to a person who walked up to me in between class
Why are you going to treat them coldly?
I feel like they have a lot of problems, and I'm tired of dealing with problems.
I understand when you do not feel like talking to someone, I feel like that sometimes too. But this person may just be trying to no you and try to be friendly to you, so I would recommend that you give them a chance to get to know you, and who knows you might acquire a new friend in doing so.
I don't want to get to know them. I want to be left alone.
You don't have to become close friends with people but it also good to try not to be cold to anyone either.