Once the mom forbade the school to touch her child, she made the cuffing almost inevitable.
Consider how hard it is to handle even a small cat who doesn't want to be handled. To subdue a fractious child without physically harming her is no picnic. And if the kid is used to getting her own way, she's not going to go quietly when someone starts clueing her in on the facts of life.
Padded restraints might have been better, if any were on hand, but compared to causing the child so much pain that she had to comply, to clubbing her or tazing her, I think restraining her with cuffs isn't too bad.
Mind you, I come to this with the esperience of being in a 45 minute battle of wills with a darling litte 5 year old girl who had never had to do a thing for herself. I asked her to take her coat off. She told me to take it off. And the race was on. 45 minutes, tears, and tantrums later, she unbuttoned her coat and joined her playmates. (Two days later she told me she wanted to marry me!)
My wife currently is dealing with a 7 year old girl who has basically wasted an entire school year for her classmates. The school's hands are tied.
And then there's the time when my wife came home with a perfect imprint of a juvenile dental arch -- uppers and lowers -- on her lower arm. I personally would have charged the child with assault and battery, but that's because I'm not a nice person.
Nope, handcuffs are modest and judicious.