There's this guy in my class and he pretends that he doesn't like me when I know he does like me. I know because last year he kept asking me if I liked him and he always stares at me and sometimes smiles. When he was my partner last year he'd do all the work and always help me on stuff. So why don't guys just admit they like the girl and ask her out?

Shyness, nerves, etc.
It can be hard to tell someone how you feel, because of the possibility of rejection.

I bet you're being unfair and don't even realize it. How? Well, guys and girls think completely differently. But even if they didn't, why is OK to ask questions like "why doesn't he just do THIS?" or "Why doesn't he just say THAT?"

You imply that him just telling you makes everything OK, and it doesn't, does it? Maybe "telling" is how YOU would do it, but does that mean him not talking about feelings is wrong? Of course not. It's not fair to expect others to behave the way that makes sense to you, and I bet you wouldn't like if guys suggested you change the way you behave to be more like them, would you?

And lastly, since your question implies the right thing is to "just tell" how you feel, then can I assume you have done this very thing? You told him? Did you? Or did you sit back trying to interpret his thoughts from his smiles and stares? Hmm?

So, do what YOU think is natural. If his stares and smiles get your attention, then he may be getting the effect he wants. If you think talking about it should be the next step, then DO IT. Waiting for others to do what YOU want is pretty silly, especially if there's a chance what you want them to do may be completely contrary to their character.

Guys don't always verbalize well. Watch what they DO. Go with that.

Stop assuming, and questioning why, when all you have to do, is talk to him, like you want him to talk to you.