Because it's the epitome of symptomatic problem solving. Europe has had school shootings. Not anymore. You just recently ran into one of the consequences.
Unless you want to pull a wuf and say the 2011 norway shooting or the Charlie Hebdo shooting would have happened in schools if the whole of europe wasn't a "soft target", we're comparing a shooting every couple of weeks to one a decade. The difference to me seems obvious: much tougher background checks and proactive psychological evaluation of students.

Yes, sometimes an armed guard can intervene and there's only 3 dead kids instead of 20. But sometimes there are 20 dead kids. Unless you can put one Jack Bauer in every classroom, that's not a very effective solution.