Currently anyone with valid identification can gain access to most school offices without being searched or screened for weapons. Once inside, if carrying a concealed weapon, someone can easily repeat the events that occured in Sandy Hook Elementary School.
A metal detector in each and every school in the USA will deter anyone trying to carry knives guns etc into a school building. These would serve to scan the students, Visitors etc. Most of the mass school shootings in the past were carried out by students. Having a metal detector will prevent a lot of these tragedies from occuring in the future. We are already used to going through them in the airports and government agencies, we can get used to them in school also.
This would help keep every student safe.