Nobody wants to stand up to these asshole parents because of expensive lawsuits and, more importantly, bad publicity. School districts don't want to deal with lawsuits that inevitably suck funding from things like renovations and school programs, not even ones they'll very likely win.
But in the long term bad publicity will be worse. If the vice principal and principal were to stand up and tell Kayli's mother sorry, for liability reasons they cannot let immobile students and those with a lot of medical equipment go swimming, they know she isn't going to accept it quietly. She's going to raise a big stink with her followers, the paper, the local news stations, and the next thing that happens is "Local school hates disabled children, won't let them swim with other kids" and they're swarmed with angry phone calls, letters, people withdrawing their kids, even people withdrawing funding for renovations and school programs. And for a long time they will be known as the school that hates disabled children and won't let them swim with the other kids, even if the disabled kid in question is completely immobile, hooked up to medical equipment at all times, and doesn't even know that she's in a pool.
It's just much easier to say "okay" and otherwise be done with it.
