Interested or not? player?

This is about a guy I don't really know. I'm gonna call him A. I was in his class last year just for 1 hours/week. He's a kind of popular guy whereas I'm very discreet and unknown. He is friend with girls who are populars.
I noticed A. only because a friend of my friend had a crush on his friend (wow! sounds complicated, no?).
And I knew from an acquaintance that his friend thought I had a crush on him too (which is totally false).
A. never does anything about it and we never talk to each other.
But last week, as I was walking in the street, I saw two guys seated on a bench and one of them was looking at me but I didn't recognize them. When I got closer I recognize A (the one who looked at me) and another friend of him. As A. saw me, he stood up and continue to look at me.
I just ignored him and walk past him without a look.
Next day, I just raise my eyes, in reflex, and I saw him, he looked at me and half smile. Straight away, I turned my eyes in another side, without understanding.
Since everything which happens, we always meet each other eyes when we come in the same place.
As it disturbed me a lot, one time, (I've vision problems to see from far) I saw a guy looking at me... no, more like staring and - as for the last time - I didn't recognize him at the beginning but when I finally recognize him, I "frown" a little like "hey, why are you looking at me?" and he reacted by frowning but with a little smile.
Since this, he only looks at me when he thinks I'm not looking and I caught him checking out my butt. Now, he just glances at me while he's exactly in front of me and quickly looks away.