The term SCH. PERIODS is an acronym for School Periods. It refers to the different blocks of time during a school day when classes are held. Each period typically lasts around 45 minutes and can be used for a variety of activities, such as lectures, discussions, or lab work. The four-letter acronym SEMS is often used to refer to these periods, with each letter representing a different type of school period: S for Study, E for Exercise, M for Meeting, and S for Special.