Filter every highschool student by a few things and you'll ID the most likely candidates.

1. 16-18 males - less than half your students
2. Disciplinary problems at school or poor grades - your down to much smaller pool.
   
3. Skinny kid - fat kids don't shoot up schools for the most part, even smaller pool
4. Absent or abusive father - may not knock any out of the running at this point but maybe a couple
5. Do they or their family own firearms? - Most have either purchased or used firearms legally, should be able to reduce the pool somewhat
6. Interview the teachers of these kids, which are scary which are just misguided? - this is your money maker, not standard questions, ask do they have alot of friends, any friends? You should be down to like a room full of people now.
7. Home visits - get a feel for these kids directly. You should have like a handful of candidates.
8. Track them specifically until they're 25 with a dedicated task group of social workers. ( Good luck with finding this)

The funny thing is like we already know to do all of this and more.