This morning, LAUSD Superintendent Ramon Cortines announced that due to a “credible threat,” all of the 900 plus schools in the LAUSD would be closed. The specifics of the threat are still a bit unclear. Cortines said early this morning that the threat was electronic in nature, and may involve backpacks. But Police Chief Charlie Beck said the threat was traced to an IP address in Germany, though the source may be closer to home. It mentioned assault rifles, pistols and explosive devices. The FBI and the LAPD are investigating. How unusual is a closing like this, and what are teachers and students experiencing?