You may want to mention in your post that Mcgonagall happened to be passing - perhaps drawn by the noise - rather than just having her appear. She'd also probably pull the two boys apart and frogmarch them both to the nearest classroom or office, along with Roze and demand to know what happened, rather than just walking off before defusing the situation. Once getting the story, she'd send the injured off to Madam Pomfrey and have a word with the Head of Houses.