Each Class must have at least one Class Section. i.e. your organization may have more than allowed kids for a specific grade. In that case you have to split the class in to two. i.e. Grade 2 Section A, Grade 2 Section B. In Ad-Din you must have at least one Class section per class to start with and can add more sections as your students count grow.
Deleting a Class Section is disruptive. This Action can't be reversed, the associated history of this Class Section will be lost. Instead de-link it from the teacher and rename it with prefix "Don't use – class section name" or as desired, or carefully analyze and then decide to delete.