Tell the first grader the starting time and finishing time of school, emphasizing that it's the same every weekday. By the end of kindergarten children are able to tell time to the hour, so even a first grader at the beginning of the school year understands the concept of time.
According to Great Schools, students learn in first grade how to tell time to the half-hour using both analog and digital clocks. Prior to the age of six, though, children have difficulty seeing the world logically and understanding cause and effect. According to 2005 Pennsylvania Teacher of the Year Nicola Salvatico, first grade is the initial time children are able to connect real world math to their own point of view. In fact, teaching scheduling such as school start and end times is a helpful way to emphasize these new math skills.
To practice time skills at home. Parents.com suggests using an analog clock to emphasize telling time and using calendars to mark important dates and events. Likewise, utilizing time words such as "today," "yesterday," "tomorrow" and "next week" help cement the concept of scheduling in a first grader's mind. Therefore, showing how all these new concepts of time relate to the idea of school hours being the same every weekday is the most effective way to explain the schedule to a first grader.