I'm not sure about that. My high school had everyone funnel into one entrance just because of where the parking lot was relative to the school. It had other entrances but no one used them normally. We turned out ok.
But if it's that much of an issue, then they can have like 2 secured entrances and not 1.
That seems very shaky, there are loads of buildings that are already designed to be very secure and difficult to break into regardless of what the attacker knows, and they're not all drab prison like facilities that people hate going into.
Secondly, even if it can be circumvented, adding more barriers that any attacker has to hurdle, any new tools and equipment that he needs to buy, still increases his risk of failure and slows him down.
And third, you're hyperfocusing on schooling shootings where they shooter 7s a student, vs an outsider. That's usually the case for high school shooting, but not elementary schools like this one. Those are the work of outside parties that would have more issues with a properly secured building.