There have been many instances where one or more school systems and/or non-public schools have banded together to form regional emergency response approaches. This can be particularly effective for non-public schools and small rural school systems lacking resources, such as school buses or mental health personnel that might be needed on a short notice in greater numbers than would be the case in a mass casualty event.
While there are such resources available from outside the region, the time to have them on the ground and the time it takes to coordinate them can be longer than ideal without adequate pre-planning. Taking the time to look around the region for partner organizations like your own can be an incredibly productive effort.