I've really liked this lad for ages, but never spoken to him because he gives me the signals that he doesn't want me too. 

He recently got a girlfriend, and I think he is showing signs that he knows I like him including his girlfriend.

She saw me and went look who's over there to a lad who's one of his friends which he proceeded to turn and look at me.

I think he knows I like him because he keeps looking at me in class, but I'm not sure if he's just being nosy as he is the type to look at everyone. It just seems at the moment he's looking at me the most.

For the past few weeks I've looked up to find him staring at me once or twice, but because I think he knows I like him I quickly glance away.

Although he only seems to start looking at me either at the very start of the class or ten minutes before the end when he's finished work.

A couple of days ago I was walking past him with my friend laughing, and according to my friend he kept his eyes fixed on while he walked past.

He is checking you out and watching your every move.  It shows he is interested in you but not quite ready to approach you and chat yet.

Are these signs to show that he knows?

No, he is checking you out from a distance, and seeing if you respond.  Then gradually he will move closer, before eventually engaging you in some brief chat and gradually building things up from there. You said at the beginning of the message he gives signals that he doesn't want me too, well that's probably because it's too soon and he wants to suss you out from a distance.  But also you say he has a girlfriend which will equally put him off, if he has a girlfriend that he enjoys being with then he shouldn't really be looking at other girls.

Is he looking at me because he's caught me looking at him?

Well, yes, but also that he wants to look back at you as well, because he finds you attractive. 

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