Will a guy talk to other girls comfortably but not talk to the one he likes?

Makes complete sense. He likes you. Guys, at least to my knowledge, get nervous around the girl they like. Ok he can talk to other guys fine but he is looking at you, that means he is thinking of you all the time. He talks well with you if he has a specific reason to because he practiced that all night and he doesn't want to mess up and look like an idiot in front of you. He is all tense and anxious when you go and talk to him, that is because he was thinking about you and you showed up and he is wondering if you somehow figured out what he was thinking about. He can talk easily online to you, that is because online you can type something, look at it, then delete the stupid joke he made, then retype it and send it to you. He can talk to other girls because he doesn't like them, he likes you! Being a year older doesn't matter, all that matters is how he feels for you. He looks at you then when you look back he looks away because he is shy and embarrassed that you caught him looking at you. Go up to him calmly and talk to him, maybe bring up hanging out outside of school if you like him back. See what develops.

He likes you. And he's making it as obvious, to the best of his ability. Most guys will be like that (not saying much to their romantic interest). They want you to view them in the best light as possible. That involves not saying something stupid or weird. So he's scared that something he says will repel you, and then he'll lose you. So yeah. Talk to him, make yourself appealing and natural in conversations with him, and he'll loosen up with time.