ISIS needs to be stopped, there is no question whatsoever about that. Any radicalism should be combated, whether it is ISIS, the Boko Haram, or Al Qaeda, or whether it is a western radical group. It is very important, however, that the UN, and not an individual country, takes the initiative to do so.
ISIS is a crappy organization and must be destroyed. The United Nations must defeat ISIS. ISIS has to be destroyed and the UN must to a better job. They should create a coalition and send in bombers to defeat ISIS fighters. They should also punish the Saudis for funding this organization.
The United Nations should definitely not organize to combat ISIS based on one 15 year old teen who attacked his teacher, even if this teen follows ISIS. If the United Nations does organize to combat ISIS, it should not be because of a teenager. Firstly, a reaction like this to the actions of a teenager would turn the UN into a laughing stock. Teenagers should be grounded. ISIS should be dealt with. But if the UN organizes to combat ISIS because of a teenager, ISIS will believe even the smallest of their actions hold power over the world.
For the UN to organize any variant of retaliation over the actions of a misguided child would be a laughable faux pas. If a 15 year old actually chose to attack their instructor for radical ideals, the error is in the parenting, not a puppet organization with little influence on the world.