Regarding the last sequence (spoilers, obviously):
I really need to see this at least once more before I can really feel like I've wrapped my mind around what I think is going on and articulate my thoughts with any confidence. That being said, I didn't necessarily take the tornado as being retribution for changing the students grade, although that's an interesting take I'll have to think about.
I just took it as being part of the bait-and-switch that was going on in those last scenes. You know, you think he's going to get his tenure, he's going to use the bribe money to pay his bills, and you see that sense of relief wash over him. You start to think it's is a movie about how God doesn't give us any more than we can handle, joy and sorrow come in moderation, all that hopelessness was just a red herring, etc. But then the tornado comes and you realize, nope - God will BEAT YOU DOWN for no reason with no warning.
Not to mention the phone call he gets from the doctor.
The way the I remember the ending being edited, the signs all pointed to divine retribution. Everything seems to be turning around for him, then he weakens and changes the grade, and within seconds the doctor calls and the tornado warning happens at the school. Admittedly we don't actually know if the doctor's news is terrible (though it seems likely) and we don't know if the kids will get into the shelter in time.
It is one hell of an ending though. The music and imagery together are haunting.
I finally saw this last night. I also didn't think that the final moments suggested divine retribution. (I actually didn't make the connection between the grade change and the phone call that immediately follows it until it was pointed out to me.)
Bad shit is raining down on Gopnik for the whole movie, and he is (or believes he is) innocent. So why's it happening? No reason. Shit happens. So then he commits a minor sin, and bad shit continues to happen. We feel like now he's being justifiably punished, but really, it's just the same shit happening. Still for no reason.