I'm sorry, but I feel really disappointed with your last chapter - the story jumped from the unlikely (Amy finding another AB/DL in Fay) to the completely implausible.
1) How on earth did Laura find out about Fay?
That needs some explaining.
2) Why were the girls not wearing their underwear in their PE lesson? I have never taken my underwear off during PE, so that makes no sense. Surely they would have had underwear on under their PE kit, or they could have just worn their PE shorts instead of the diapers. I don't see why they would choose to wear diapers instead of PE shorts?
3) Most people can hold their urine for the length of a school lunchtime, which is usually only an hour or so - I don't think they both would have wet their diapers in the time their captors held them down.
4) Amy's messy accident is bordering on the ridiculous. If she has never had an accidental bowel movement in class before, why would she suddenly have one on the day she's wearing a diaper? Again, most people can hold themselves for the length of a lesson, and most teachers would let a student go if they were really that desperate. Even if they didn't, most students would rather run out of the classroom to the toilet without permission than just sit and mess in front of the whole class.
5) Where were the teachers in all of this?? I know a lot of things go past them in school, but I think they'd notice 2 of their students suddenly being dressed in diapers, writing a speech about diapers, being dragged from the toilets to the spot the teachers couldn't see etc etc. Teachers aren't stupid, and students aren't always that clever. I think they'd soon notice this torment happening to two of their students, and even if they didn't a student would probably be dumb enough to mention it in a teacher's earshot at least once during the day.
6) Why did the girls even turn up to school when they knew everybody knew about the diapers? Why didn't they turn around on the way to school and go straight back home. Their parents know about their diapers, and each girl has changed school due to bullying before, so wouldn't their parents have understood?
I thought your story started quite well, but it descended into a series of implausible events offered with explanations that were equally implausible. I also felt the ending was incredibly rushed. It is possible to have a story in which the implausible does happen time and time again, but this needs to be executed sensitively so that the implausible doesn't actually appear so in the setting of the story. That means the writer has to take their time in setting the scene for each unlikely event, and in offering the explanation for why it happened.
So for example, rather than have the girls underwear stolen during a PE lesson (when they would still be wearing underwear & PE shorts, which could have gone under their skirts) you could have had them playing a muddy game of soccer as part of their PE lesson. This would obviously require them to shower, and whilst they were in the shower their clothes could have been stolen by their classmates. Their classmates could have agreed to only give back their clothes on the condition they put on the diapers. Just thinking a little harder about the explanation you offer for the events that unfold in your story makes a real difference for how the reader enjoys your story.
As someone else said, what you actually write is written quite well and you have a good writing style, but your writing still feels too rushed and too many events unfold with little or no explanation, often explained over just one or two lines.
For your next story I suggest you take the whole thing much, much slower. As I said previously, don't feel afraid to take your time introducing your characters and setting the scene - don't be in too much of a rush to get to the exciting part with the diapers if it is in detriment to the rest of the story. Most readers like to feel like they get to know the characters over the course of the whole story - yes, we like to read about the diapers, but most readers also want the story to delve a little deeper than that. Think about how your characters are feeling and what they are thinking at each point of the story, and try to include some of their thoughts and feelings in your writing to help your readers get to know your characters.