The kirpan is supposed to be a defensive, not an offensive weapon.
Surely the solution is for the authorities to insist that if Sikhs want to carry one they should blunt the blade, as apparently they do in California.
Some regulations allow the kirpan under certain restrictions; for example, rules in some Californian schools require that the kirpan be blunted and riveted into a sheath. This prevents any possible use of the kirpan as a weapon, but still allows it as a physical symbol of faith.
www.absoluteastronomy.com/topics/Kirpan
Apparently in this case the school did offer this compromise but the boy's parents refused. In that circumstance I don't agree with the retired judge that their views should prevail.