What if a guy does not say hi back to you when you say hi?

Usually because they dont like you, although they pretend ALL the time. I get that all the time, and sometimes its because they think they are the coolest people in the school, and they're too cool to talk to you, and if they usually say hi and then they dont when they are with a certain person, they arent a real friend. Or they are trying to fit in and want to be a "cool, popular" kid. Most kids that think of themselves as highley are really idiots who just do that to fit in and feel like people care. Just blow it off and dont worry about it, just have your own friends that are really loyal, and actually say hi.