On cable, two returning shows for the teenage set: "Degrassi: The Next Generation" (The N at 8 p.m.) catches up with the gang, now in college except Sean and Ashley, who are stuck in the 12th grade; on the second season premiere of "South of Nowhere" (The N at 9 p.m.), Spencer accepts her sexuality and Ashley (not the Ashley of "Degrassi") copes with the death of Dad.