I wish Rachel would get a bit more content. I don't mean the 'fit the dildo inside' thing, which seems trivial now. I mean a bit of character development. She's been using the pet/submission thing to give herself more confidence and peace of mind, so it would make sense that she would seek out more of this in a more stressful time.
Perhaps some bad news from home prompts her to show up at the Headmaster's home for reassurance. The Headmaster, in his usual sick fashion, could demand some compensation. Not sex, which he already has, but ordering her submit more completely by becoming his housepet, and completely obedient while in his home. She would find this initially shocking but would soon realize it's exactly what she wanted. Things like this work as long as the characters retain their humanity once the fantasy is over.