Seems Brenda Spencer in San Diego, 1979, a la "i dont like mondays", always gets the nod for this, but the first in NA actually happened in Ottawa, Canada. In 1975, Robert Poulin killed one and wounded five at his highschool with a shot gun, then turned the gun on himself. This after raping and killing his girlfriend. A book "Rape of a Normal Mind" was written about it.
>>> Like most crime in Canada, it got covered up at the time.