I suspect a combination of multiple things here.
1. Very disturbed child. The article says as much: "the staff in a program for children with behavior problems like him." Kids don't normally just pick up desks and throw them, and show no fear of the cops.
2. Mother who clearly has no idea how to handle the kid and his very serious problems.
3. Possible issues in the home with abuse, drugs, alcohol, etc.
The kid obviously posed a threat serious enough that the teacher had to barricade herself and the other children to protect them from him. He was wielding a weapon (albeit crude) and was exhibiting violent behavior sufficient to be a real threat. He also wasn't calming down with either staff specialized in working with kids like him or when the cops talked to him.
I don't think the cops had any choice here, and it looks to me like they used the minimum force necessary to stop him from being a threat. If they'd tried to use physical force to stop him, not only might they have hurt him, but they might have gotten hurt themselves if he kicked them, bit them, and so on.
The kid clearly needs a medical and psych evaluation, and mom needs help learning how to parent him properly, and the child protection service in that state needs to investigate the home situation asap to see what's going on, because I smell something very foul just like Qui-Gon Glenn does.
By the way, where's DAD?