Most schools have systems to control visitor access

From the National Center for Education Statistics: An overwhelming majority of public elementary and secondary schools in the United States have systems in place to limit access to facilities.

Based on data compiled in 2007-08 from public school principals, the National Center for Education Statistics compiled this data in its Digest of Education Statistics 2011:

Security measures present in PUBLIC schools, 2007-081 Controlled access to buildings (doors locked or monitored) 88.8% 2 Security cameras

51.8%

3 Controlled access to grounds (gates locked or monitored) 44.9% 4 Daily presence of police or security 27.2% 5 Random dog sniffs 25.0% 6 Random drug sweeps 14.8% 7 Badges required 7.5% 8 Random metal detector checks 5.9% 9 Daily metal detector checks 2.3% Source: Digest for Education Statistics, 2011