Teachers often look at the school year as a giant math problem:
(students needs + interests + learning preferences) X (required standards) X (effective educational practices) + (teacher prep + planning + professional development) / (energized hours in a day X (school days in a year + added days for professional work) = optimal school year.
It's a complex problem with many more details than listed above. A school day is often looked at as a math problem too:
(students needs + interests + learning preferences) + (daily learning goals + standards + routines) + (daily unknown factors: skinned needs + social conflicts + school-wide additions + unexpected events) = typical school day.
So teachers choreograph a multi-modal, positive-momentum, varied-materials and activity, researched based, patterned day to best teach all, and sometimes the equation doesn't work exactly as planned.
That's when common sense, flexibility and effective leadership and facilitation come to play. It's okay to choreograph the day, but it's also imperative to leave space for the unexpected and unknown. Further, it's important to realize that the spontaneous and serendipitous elements in a school day are sometimes the most valuable.