Head to the school and enter the school from the side. You'll find the goth kids there, hanging out. Talk to them and another sub-quests will become available. This will also lead to yet another sub-quest that's connected to the main quest.
After successfully recruiting the goth kids, you’ll have to decide which side you'll assist; Kyle's elves or the humans led by Cartman. After reporting to Kyle or Cartman, more cutscenes will follow. Both groups will end up meeting up in a battle at school. Depending on who you sided with, Cartman or Kyle will be added to your Friends List as soon as the next quest starts.