Wouldn't it depend on how much the student resisted as to the cause or severity of the Injury? Even a non violent restraint can cause injury if the opponent resists strongly.
Still begs the question how a trained adult was 'forced' to break both femurs in a sixteen year old disabled boy to control him until help arrived. And how was the aide's life so very much in jeopardy after the first leg was broken that the second leg needed to be broken as well to stop the student from - what? Not running after the aide.
Resist a Nikyo hard, get a sprained wrist or broken forearm, maybe dislocate a shoulder. But what Aikido technique breaks both legs?