Hey, congrats! That's really good work!
Well, there are a lot of ways to inflate a NaNo word count. You could start on your second draft of your novel. You could go back and add some descriptions and dialog. You can add a prologue of what they were doing before the story. You could write an epilogue (the only time I "won" NaNo, I had to write an Animal House style epilogue that told what each of the main characters wound up doing with their lives after the story was over -- I tend to write short; my longest stories end at about 38,000 words.)
You could start the sequel to the novel. You could start a completely different novel if your teacher says it's OK. You can rewrite a scene -- keep the old one, and just put a strike-out font through it. That way, the old scene and the new scene will both count as words.
If the story is done, the story is done. I mean, there's really not much you can do about that!
NaNo org itself has some good pointers on how to get more words; do your research, so if your teacher complains, you can say, "Well, NaNo says it's OK right here."