I think I am crushing on my ex best friend

Last time we talked as real friends was 2 and a half years ago

We go to the same school so I have always neglected him for the past 2 years because we ended our friendship tragically...
I went back to school a few days ago and he's in almost ALL of my classes

he kept staring at me, and once I caught him doing so but he didn't look away.

I don't know what to do.
