Buffy only stopped being in Walsh's class because Maggie tried to kill her and then was herself killed by Adam; Buffy had referred to Riley as her 'boyfriend' before that point and they were definitely dating, even if they hadn't yet consummated the relationship.

When I researched TAs for a fic one of my sources was a website giving guidance for English graduate students taking a position as TAs in American universities; something that was stressed over and over again was that if you even asked out a student in a class you graded then you would be immediately booted out of the university and kicked straight back to England, probably in the baggage hold. Riley was guilty of gross impropriety either through lack of morals or extreme stupidity - either of which makes him unfit to be a partner for Buffy.

I have written fics in which Riley is a good guy - but then I've also written fics in which Warren is a good guy, and I actually find that easier.