Angel's a vampire and it was day time.
From Buffy's perspective, she wants him to stay because she thinks she can take care of him, because she's the Slayer and he's just a well-connected human. If he leaves her house, at night, to walk home alone, he'll be attacked by the Three. If Buffy leaves with him and walks him home, they'll either both be attacked and have to run back to Buffy's or on to Angel's, or Buffy will manage to get Angel to his place, and she'll be attacked on her way home. It's already been established that Buffy and Angel together can't defeat the Three, so Buffy won't survive on her own. She's rather know that Angel is save at her place, so that the two of them can go out together - maybe with Giles and the Scoobies - the next night to finish off the Three.
The fact that the Three have been killed by this point is moot because Buffy doesn't know that.
From Angel's perspective, there is no opportunity for him to leave. By the time they get to Buffy's house, it's late and Buffy is the Slayer, so Angel knows he'll wake her if he tries to sneak out. By the next morning when she wakes for school, it's too light outside for Angel to even think about going anywhere, and Buffy's house has no sewer access that we know of. When Buffy gets home from school, it's dark enough for Angel to leave, but he's polite and waits for her to come and see him, which is unfortunate, because by that point he can't control his feelings for her anymore and kisses her, revealing himself as a vampire.