According to an August press release, the U.S. Federal Emergencey Management Association (FEMA) and the Federal Communications Commission (FCC) will test-deliver a nationwide message to all mobile devices, radios, and TVs at approximately 11:20AM, PST.
Kennedy students will be in Period 3 when the message arrives. No follow-up replies or actions will be required of smartphone owners at the time of the message.
In their joint announcement, FEMA and the FCC state the test is meant "to ensure that the systems continue to be effective means of warning the public about emergencies, particularly those on the national level."