What leaked wasn’t the manifesto.
What leaked were a couple of pages from her journal where she expressed self-loathing, hatred for the targets her indoctrination indicated, and a set plan for the school on a specific date.
That set plan for the school on a specific date has no explanation without more documentation, so it is entirely possible it was intended to be a coordinated strike among several schools...or she was activated as a sleeper agent with the promise that it would be a coordinated strike among several schools.
Wow! That would explain the complete blackout of information. Feddies don’t want to be exposed…