zero period
 n.— «The new schedule has seven 50-minute periods, and extends the school day by 15 minutes. Before, the high school day had six 55-minute periods, but many students took an extra class —- commonly called a “zero period”—before the regular day began.» —“High school reform arrives” by Carolyn Norton Herald-Sun (Durham, North Carolina) Aug. 23, 2006. (source: Double-Tongued Dictionary)

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