My Ideas

1. Ban assault rifles
2. Create a special department within Homeland Security or the FBI, charged to monitor and act on citizens who pose a potential risk.
3. At a national level, have mental health professionals lay out effective steps to provide care to those in need. Staff and fund state and local governments to carry this out.
4. Expand background checks on people buying guns
5. Metal detectors for high schools, much more highly restricted access to schools
6. A coordinated state, local and federal campaign to increase awareness of what leads to these situations. Something along the lines of "if you see (or know) something, say something"

Even if every single of of these are enacted, there's no 100% sure way to ever prevent a mass school shooting again. However, I believe incidences can be greatly reduced.