At precisely 11:51 a.m., the first seven students busted through the doors of Southern Regional High School’s 11/12 building, masks on as they headed for the row of buses awaiting in the south parking lot. Southern Regional School District Superintendent Craig Henry and a dozen or so other administrators were on hand to direct traffic as needed.

“Everything today went exceptionally well,” Henry said moments before the first wave of some 800 students left the building for the first half-day of…