And as pointed out it doesn't always help if the officer does nothing like the Florida shooting.
2. Then there is the issue that they can't be everywhere at once and a shooter can still get in to the school.
3. Someone mentioned the locked doors. This is huge. There should be one entrance used to the school. Which is a problem at older schools that have multiple buildings like detached gyms, locker rooms, band halls, etc.
4. All schools need the locked outer door and an interior vestibule door that is locked.
5. Then go a step further and the glass on both doors and windows next to them need to be bullet resistant. I know of plenty of schools that have the two door system but both are just regular safety glass. Hard to break with a rock, chair, etc. but a gun would be able to shoot it out.
6. Panic button at the front desk.
7. Front desk person armed/ safe with locked gun. Or at minimum something like a can of bear spray.
8. Armed staff
9. Lots of drills. Currently in Texas schools have to do two lock down/active shooter drills a year. It should be more. With practice on what to do.
10. All classrooms should have devices used to jam the doors shut. There are a lot of options for any type of door.
All of these don't need to be done but at least two or 3 would help a lot.