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Many Synagogues in the US, the ones that can afford it, have had tight security for decades. I've read estimates from 10,000 to as many as 25,000 military and police to protect the Synagogues and other institutions of 500,000 Jews in France. It's potentially a very large and very expensive proposition. And no, while congregants should be able to be armed, that's not the same as the 24/7 security necessary in Europe. With Church vandalism and attacks running at a rate of 600-1000 per year in France, the Sunday people are next. Probably better we don't import the culture.
The congregant who is armed is only protecting/defending him/her self and family. Protecting the whole group is not within his/her purview. And I would say the same for a school teacher, or school office worker.