"Why does this guy talk to me when we're alone, then ignore me if others are around?"?

There’s this guy I like, and we used to be in the same class for 3 years. Now we only have one class together, but he hardly ever acknowledges my presence, especially when there are other people around (e.g. his friends). However, whenever I’m alone, such as walking along a corridor, he will always make his time to say hi and chat to me- and I know for a fact that he doesn’t do this with everyone. Once I pretended to not see him and I could see him staring at me from the corner of my eyes.

I’m so confused! Please help :(