Use School Calendar tab to define the year calendar at school level. Add holidays and vacations specific to a school that are different than a district calendar.
Use School Report Periods tab to define periods for attendance reports. These periods are only used for reports, so they may match grading periods, terms, or whatever other intervals are helpful such as 40th and 100th day. Like School Calendar, School Report Periods can either use those defined at district or override periods at school level.
School Calendar