I know it's an out there eruption, to have it in the middle of class, but what can I say, I'm an exhibitionist at least when it comes to emotional outbursts. I've always wanted to write a scene like that. Not only write it either, but when I get really pissed off at anyone, I imagine how I would go off at them, how I would humiliate them and get the upper hand, etc, even though in real life I would do nothing of the sort. But it's good to dream lol.
With the decision making, in this I consider Snape to be very very protective of Harry, it's what he's been doing for all of Harry's life, but now that he's old enough to make his own decisions, Snape still can't let go of trying to make them for him, to do what he thinks is best for him. And in the Harry has to make it clear that he can make his own decisions.