Buzzers for the front door. Security cameras. Bulletproof glass.

These are among the physical reinforcements that have become more common in schools across the country in the five years since the tragedy at Sandy Hook Elementary, as more and more schools try to address security concerns.

Experts say the Dec. 14, 2012, massacre in Newtown, Connecticut — which left 20 children and six staff members dead — prompted the wave of alterations to school buildings.

"After Sandy Hook, you saw a tremendous increase in the number of buzzer systems," said Dr. Amy Klinger, director of programs and co-founder of the Educator's School Safety Network, a nonprofit that supports safer schools. By requiring that visitors be buzzed in by a receptionist or other staff member, schools say they're adding a layer of control about who can gain access.

Adding security features comes with challenges, though: They can be costly for cash-strapped schools — Klinger said the buzzer system alone can run about $5,000 — and they can make school feel less than inviting.