Because school doors are usually made of safety glass, which does not shatter.

"Adam Lanza shot his way into Sandy Hook Elementary School with a volley of rifle shots to a glass door"

- [link to www.thetimes.co.uk]

"Dawn Hochsprung, had recently installed a new security system in which the school doors were kept locked all day starting at 9:30. But Lanza had apparently shattered the glass in a window or door."

- [link to articles.washingtonpost.com]

"At around 9:30 a.m., Lanza approached the school building and, unable to open the locked doors, shot his way through a window near the front office to gain entry to the school."

- [link to abcnews.go.com]

There is inconsistent information and it would be an easy shot to provide proof that this happened.