[CalChris]

I side with the student. This is beyond a jerk move.

To be clear, the beyond a jerk move part is doing this in front of the class with such flimsy evidence. I have see this before. It happened to a black woman friend. In this case the professor's flimsy evidence was a shift in tone which happens in editing all the time.

That said, hence is an adverb and not a conjunction. Hence there is no comma after hence. BTW, this punctuation mistake argues strongly against plagiarism.

[hooloovoo_zoo]

This is not a uniformly accepted rule. Commas are common after adverbs such as therefore, nevertheless, and hence. There is no mistake here.

[CalChris]

You might be right. She may have used hence in the conjunctive adverb sense.

Having looked it up a bit, it still goes both ways. The argument against the comma is that if hence is a transitional word then you would omit the comma. If you feel the need for a pause then you would insert it.

So it's a fielder's choice and not a mistake.