Guys! why would you act like a stranger with your girl best friend in group? :/?

[Rythm Of Crazyness]

First thing i can find that you still have feeling for him or love him.
He seems obviously embarrassed to come back in your life as you both had history.. One thing i shall like to say as i am also a guy he may not be with you as he is with lots of female friends so he is almost behaving same as with others. And a little more i shall say if there is anything still between YOU and HIM go and talk as misunderstanding is very dangerous thing.
Be practical here and don't get too much in emotions of past and he stares you in class like things .You know guys do lots of time pass / flirting with girls.So be aware rest of all you are intelligent.
Best of luck