Too many comments about the one-sided ACLU picture painted by the OP. I see no police or school racism here. The parents of the threatened kid might be racist (who knows), but there's no way to tell, since their kid was actually threatened, with proof to back it up.
The parents of one of the kids threatened in that drawing contacted the School and insisted that the school refer the child it to the police (ergo school not racist). If only they would have done that in Columbine....those kids were pretty vocal about what they intended, but people blew it off.
In my previous career, people that were either dangerous to themselves OR others were routinely cuffed until things could get sorted out. In some circumstances, we violently took them down, yet in others we created a ruse, and once safely away, cuffed them as gently as they (the person being cuffed) would allow. There were a few times that it wasn't an option, and sometime people got hurt.
Age isn't a factor when violence is concerned. Guns are the great equalizer. Imagine all the ways a mad, tantrum-throwing 10 year old (claims was ADD) with a gun might behave, and the answers are endless. They handled the restraint portion without tact, and hurt people feelings. Officers had no way to know is the girl had access to a gun, knife or anything else.
The police weren't wrong for arresting/cuffing the kid, just not very experienced I'd imagine, at arresting kids at a school. If the same thing happened at my place of work, the same response would have happened.
Train police on how to respond to school incidents and chalk it up to experience learned. Then get all the involved kids and parents a Cognitive Behavioral Therapy.