GUYS: what does it mean when you look but then dont act?

I have a guy in my class (im in 10th hes in 12th) and sometimes we flirt sometimes we dont and i do have a boyfriend and he knows but we are doing bad (we both are talking abiut leaving but the othee guy thinks were great) but sometimes the guy i flirt with is just not responding he wont flirt and doesnt say anything and some days he does talk and seems to loom at me... why is he up and down with me