zero period
n.β Β«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 β- commonly called a “zero period”βbefore the regular day began.Β» ββHigh school reform arrives” by Carolyn Norton Herald-Sun (Durham, North Carolina) Aug. 23, 2006. (source: Double-Tongued Dictionary)