grammar - What is going on in this sentence?

I was helping my brother study for the SAT, and we came across this sentence:

While it was different from all the other classes he had taken, Eric was unhappy with his psychology class.

The answer was that there are no errors in that sentence. But my brother thinks that the noun following the comma should be consistent with the "it" in the first clause. I have come across sentences of this form in the works of well-known writers, but I do not know how to explain to him why this is correct.