As someone who oversees security on a University campus I can tell you this (regardless of whether I agree or not with lockdowns, campus carry, etc) - one of the purposes (benefits?) of lockdown is to provide more barriers to slow the perpetrator down until the police get there. Yes, it easy to blow through a glass door, but navigating through hallways, especially - and classrooms, slows the guy down and presents him/her with more decisions. It also obstructs lines of vision (I know, not with glass) and helps challenge navigation. The real effect of these measures may be minute, but I think in these situations, anything helps.
Further, polics have altered tactics in responding to school shootings. before they used to stage, call SWAT, secure the wounded for the medics, etc. In the police departments I deal with, the response is for the 1st officers on scene to immediately enter the building and focus on nothing other than neutralizing the threat.
Sorry for the tangent