No. We recommend going through all 30 units (or lessons) of Essentials at one level, in order to introduce students to all the spelling rules and basic phonograms sooner. You then have the option of going back through all 30 units on the next level up to review these concepts and apply them to more advanced words.
Most customers find that repeating Essentials at the next level up moves more quickly than the first time through, as the core content of the lessons will be review. Students are able to review the core content and apply the concepts to more advanced spelling and vocabulary words. Level C adds in many of the Latin and Greek roots in the vocabulary sections as well.