You cannot delete or skip lessons that are in the Lalilo progression. However, you can assign a new introductory activity, so the progression adjusts to their current needs, or (if you have the Complete version) you can assign specific lessons to students who need to work on special exercises.
If you assign a new introductory activity, students will take it when they log back in, but it will look like regular exercises on their end. On the Lesson View, their validated (green) lessons will remain validated, but their striped tiles will update to reflect the results of the new activity. After the introductory activity, students will have a new starting point in the Scope & Sequence.