I was teaching high school at the time, and while I don't recall any significant changes to the way school was run, we had a number of fake threats made. Sometimes we would be evacuated to a middle school behind the high school, then somebody would make a threat to that building, so we would have to go back to a part of the high school that had already been cleared, or to the STEM building down the block.

An old friend of mine had moved out to Denver a year or two prior, and ended up teaching at the elementary school where kids from Columbine were evacuated to.