Lesson cannot be completed in standard time

Samuel is teaching a science lesson. Students enter the classroom, sit and he begins the lesson. Not all students have their workbooks with them, so need to leave their seats to get their books. Samuel must wait for them to return to their seats and re-settle the class before continuing. During instruction, Samuel adds an activity for the students to complete in pairs as he believes they will find it interesting. They take a long time to organize themselves and start work. Samuel then quickly transitions into the independent work activity indicated in the lesson script as he is running out of time. Students are confused and many do not start the independent work before the end of the lesson. Samuel resolves to continue the lesson during their next timetabled lesson.