I was also thinking about the place in which he posted it. I'm at work, but it was a school website with the explicit goal of bring things like that to light. That sort of brings into question whether this school has a problem with this already that warrants a website like that.
http://obiemicroaggressions.tumblr.com/ A much longer version of the letter is on the site. The white guy seems to be confused by the situation and hammering out a lot of identity problems. It's a fascinating read, mostly because I can identify with his confusion and self aware lack of understanding.
Or does the existence of a prominent outlet like that create more situations like this, which would otherwise be resolved in private? I find it incredibly rude that the 'aggrieved' student's first course of action was to publicly shame and condemn their fellow classmate in front of the world, instead of privately expressing their concerns. They certainly do not look better for it.