Maybe bullet proof glass is not possible in all schools, but at that particular school, he shot out the glass in the door to gain entry. Bullet proof glass would have prevented that. However, since his mother worked there (I believe I read that), he would have known about other possible entry points. It's hard to say for sure until there's been time to process what happened. In the meantime, however, schools are expected to continue to function and parents want their children to be kept safe.
In recent days since the incident it was revealed that in fact his mom did not work at the school & his reasons for choosing it are unknown at this point.