If it was one of the students, they're keeping quite mum. Not even a peep. That leads me to believe it wasn't - such a thing would surely get around quickly, and Carrow does have his supporters.

I shall be honest: I suspect it was the centaurs - they don't like him any more than most of the students do, and they're more than large enough to overpower him. As for Black, I can't see how it could be anything but hallucinations. I am tempted to let Poppy doctor his wounds, throw up my hands, and point out that people do get what they deserve.