Guilt
Cassidy knows why he's been called to meet with one of the adults. They're going to throw him out, and he can't blame them. The other students treat him like he's got the plague, and he hasn't seen Tina in days. Maybe she heard about Jake and doesn't want to see him now, either. He couldn't blame her.

No one wants to be friends with a murderer.

They're going to send him away, so he's packed the one backpack he owns,  one of the first real things he's ever owned in his life. He doesn't know where he's going to go, or how he'll live, but he'll have to figure something out.

Still, he looks like a man headed toward the gallows when he walks into the office.