As a former teacher and currently a university supervisor working with student teachers at their K-12 schools, I have observed that during lockdown drills, teachers have to find their keys and then go out into the hall to lock their doors. This takes valuable time and exposes the teachers and their students to additional risk.
I would suggest that all classroom doors have deadbolt locks that can be locked from the inside, but are easy enough to open in case of fire. I do not know if this is against fire code, but it seems to me a rather simple thing that may save lives in the future.
An additional thing that may help is to replace the classroom doors that have glass panels with metal doors and install wide-angle peepholes or cameras.
As a nation, we all are grieving for the families of those who were taken from us too early. Hopefully, the time is now to do more to protect our schoolchildren and the public.