This is just for RJ and all the fans that what to know how this teaching came to be. . . Buffy and Angel had sex, near where Angel then left. That was the second time that Angel lost his soul. The slayer got pregnant and in an unseen episode, she killed Angelus, but for the show 'Angel' to go on, they didn't show that one episode.
In the Angel series, Angel loses his soul once again and makes his son full vamp, seeing as his was already half, then regains his soul again and kills his son. I don't know for sure but I think that the son was named Robert or something.
Anywhoo, this is were the timeline changes. Angel lost his soul, making love to Buffy, Willow spells it back so it won't ever leave again. The child that was conceived was still born.
This is where I bring in Harry Potter. When the child dies and Buffy and Angel are still weeping, the get a letter, by owl mail. It's is asking both of them to teach at Hogwarts, Buffy is a witch and Angel is hunter. So they get there just when the year starts and they've gotten over the dead child. The reason for that is that she just found out that she is pregnant again and the child will be born somewhere in March, which means that she is two months pregnant.
That is how I figured this story out.