Why do they look at me and shout his name?

I always walk past my ex boyfriend at least once a day at school, but whenever I do, all his friends shout his name so that he sees me and he stares at me and they all watch me walk past. It freaks me out because their always staring at me if I'm anywhere near them. His best friend also has his eyes fixated on me wherever I go. He gives me a weird look and try's to hear what I'm talking about with my friends. What could this mean?