Metal detector at this single entrance. Can we posit a scenario here? A violent teen with lethal firearm under trench coat will: a. Wait patiently in line with 1,000 or 2,000 fellow students or b. Blow the metal detector guy away before storming the school in a hail of bullets.
Neither of those, as he has sitting ducks in a row for easy mass assassination. He just sits and waits until he is ready to open fire.