Their friends

I know that Eric and Dylan's friends were expelled from Columbine for the next school year (I assume the seniors weren't allowed to attend graduation ceremonies either) but where did they go?
How did they determine who were "friends" and who were not?

Who was expelled? I know they told Brooks not to come back but they also told him he'd still graduate.

I don't know if anyone was exactly "expelled". A few of their friends had already graduated (founding TCM members like Joe Stair). Some others personally felt that they could not return to Columbine. I don't know about Nate Dykeman, but I'm pretty sure Chris Morris did not return, and we all know Brooks Brown was asked not to come back.

The school had no right to expel the friends and associates of Eric and Dylan so they threw in a sweetener to make them go away: early graduation.