Building checkpoints to protect our children in school

Most of the schools I've been in have the same protocol for visitors. Check in at a front desk or office and receive a visitors pass before proceeding into the school. Anyone can bypass this protocol by walking right past the front desk. I'd like to propose a simple solution to protect our children in school. Build a wall that isolates the entrance lobby from the rest of the school. Anyone entering the lobby would have to be buzzed in to the main interior of the school. By segmenting the school into zones, we could give our children enough time for authorities to arrive. The murderers from Columbine and the Connecticut shooting arrived after school had begun. A simple isolated checkpoint could have been enough to prevent these massacres.