That false smile fades, but the little frown that follows probably isn't much better. It's still so easy to fall into the student role to Callaghan's teacher, and like any dedicated student, he can sense the displeasure hidden beneath that acceptance. That resignation. Maybe it's not specifically directed at him, but it's definitely there.

A small part of his mind nags at him, reminding the rest of him that he doesn't have to feel this way. Not for this man who still seems so unwilling to try and fix the mistakes he's committed. Callaghan isn't Tadashi's professor anymore. Popular opinion would probably insist that he shouldn't be Tadashi's anything, and yet...

("And yet" seems to be a running theme with Tadashi at this point.)

He nods again, a new smile forming that's much closer to his normal fair, if not a bit chagrined. No need to mention the lies he'd had to tell the school board to get himself readmitted, or the stares he gets from certain lab mates who aren't in the know. Or the fact that he hasn't managed to make a single major improvement to Baymax since his return.

"Uh... yeah. I missed a lot, though. I've got some catching up to do."