True, but you'd still have to enter through the front entrance, and the front desk can see if you are a person who should have a key.
If prepared for this scenario, especially with some armed staff, I'm comfortable that none of these weak psychos who need to attack children to feel strong would be brazen enough to try and breach it.
And if they did, they would meet sufficient resistance to end the attack.
Sure, it's possible. But there is a significant staff presence outside when school starts. And they notice people who are out of place. Even when I enter with my kids they make sure to address me.
Nobody is sneaking in there with an AR unobserved. The only feasible way would be with a hand gun on your person or in a backpack...