Schools increases the security after Columbine, and more are starting to get extra security after Red Lake shooting few years ago. Red Lake school shooting is 2nd worst school shooting in the nation.
As for first HS I went to, I heard they increased security, dismissed security liasions that I knew, hired "rent a cop", and only seniors would go out to McDonald's for lunch (During my time 10th to 12th grade can go out eat). Seniors only have ID similar to my work ID to increase the entry to school buliding during lunch hours. If a senior let underclass or 11th grade accompany them, they will lose that privilege.
2nd HS I went to (actually graduated from) only have staff seated in the front with desk, where all visitiors can sign in and out, also have police officer from local PD help with lunch hour.
As for MN school for the deaf, they have their own security guards (thats what I heard).