I can't get over how much this reminds me of that one episode of Strangers With Candy. That racism episode gets the nod, because this one probably doesn't have Principal Blackman dressed like a squirrel.
The locker reveal was pretty awesome, but I can't help but remember the Strangers With Candy episode where Jerri tagged the wall and Principal Blackman called in Tim Meadows... so I laughed.
Laura spray painted the word "nigger" on her own locker to bring attention to her self-serving campaign to start a black history class. Urkel is her bright but naive and easily manipulated pawn in it all who doesn't need to fill out a college application with a huge number of extracurricular activities.
This is the last paragraph of the plot synopsis for this episode on IMDB:
Meanwhile, Carl has ignored his family's recommendation to save them the trouble by opening up his wallet and buying another vacuum cleaner. He attempts to fix the old vacuum cleaner himself since it's cheaper and hadn't have too much success
I think the letter was a sincere, if not badly-worded, recommendation to learn more about black history by visiting Africa. The racial slur spraypainted on the locker was just a terrible coincidence.
It was really a very special episode about dyslexia: low self-esteem because of dyslexia drove a kid to start picking on gingers, but not only couldn't he find the right locker, he couldn't spell it right either.
Kids with dyslexia can be a handful but they need our patience.