Why does this guy act like he doesn't 'see' me, when I know I've seen him staring at me before?

There is a guy that I see around a couple times a week during a class I have and can't figure out if he likes me or not. I catch him staring my way every so often, I think I've even caught him checking out my derriere but when I see him outside he completely ignores me (or at least acts like it) even when I walk near him. I know he talks about me to his friends but I'm not sure what he's saying, maybe he's saying something mean. Anyways even if he's coming towards me I even look at him to see if he's looking and maybe give a smile but he just keeps on walking. Am I making him nervous? Or is he just a jerk, who wants nothing to do with me? If he does like me how can I talk to him or at least make him feel comfortable? I just don't want to make a fool of myself going up to him and start talking if he wan'ts nothing to do with me.
Why does this guy act like he doesn't 'see' me, when I know I've seen him staring at me before?
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