That made me curious, so I looked.
On Google Maps, its 11-13 minutes from the County Sheriffs office to the school.
Taking in time for the cal, the dispatch, the reaction time, the drive, the set-up.....20-25 minutes would not be unrealistic.
Now, my local school of 400 students, is about 10 miles farther than that.
You know the swat team would be driving > 100mph
just because they could, LOL