The Canadian high school drama ''Degrassi: The Next Generation'' built a rabid Stateside following with what creator Linda Schuyler calls its ''brutal honesty'' in tackling topics like drugs, rape, and homosexuality. But The N, the cable net that airs ''Degrassi'' here, has found one plot too honest for U.S. audiences: A two-parter in which Manny, 15, discovers and terminates her pregnancy will not air. The N exec Meeri Park Cunniff says it's a matter of tone: ''It's a serious episode and the summer [schedule] is all lighthearted.'' But message boards are full of protests, and Cunniff concedes that The N might eventually air it. Perhaps there's cause for caution: Preteen self-mutilation at one Canadian school was linked to an episode about ''cutting.'' Still, Schuyler is ''very proud'' of the abortion episode, adding, appropriately, ''It's not the right choice for everybody.''