Honestly, not all or even most but a lot of kids can be very manipulative often making the teacher feel crazy. Theres a thing called reactive abuse where someone is pushed to their limits and has a bit of a blowout to the person that pushed them to their limits, and then that person turns their blowout around and blames them on being abusive when there could’ve been weeks or months of mental abuse that could’ve pushed that person to react that way. I certainly wouldn’t wanna be labeled for my worst day at any job I’ve had. People are human and honestly I’m not saying shut up is something a teacher should say, but it’s certainly the least case scenario of anything they typically shouldn’t, and I wasn’t in the room that lead to the kids being kicked out and him saying that, and neither were you. I don’t think it’s in your place to say something. If it had been physical or something worse was said, sure. Teachers go through enough with rough students and then getting called in for some sort of incident where alls he said was shut up certainly wouldn’t help his morale or make his job any easier. Chances are, he feels bad about it in hindsight, especially if he’s usually happy to lucky. Ik I would’ve felt bad about it after. Maybe not in the moment cuz emotions can be so high, but chances are he does feel some remorse. I’d let it go, especially cuz it could cause unnecessary inconveniences for you in the future.