Do this in an elementary school.
Have some kid drugged up on pscyh drugs think he is volunteering to help.
Tell the kids to close their eyes when they leave so they don't see anything.
Bring in some nuns in masks to shoot up the school and the kid on pscyh meds.
Have an active shooter drill going on close by at the same time.
Blame everything on guns, since the "shooter" is dead no court cases.
Lock down the location where this happened and do not release any pictures or video of what happened.
Blame everything on guns then bull doze the school to rubble so no one can actually see any evidence.