Yes, this has been a problem that we are working on. If you haven't started completing lessons yet, you can clear the Assignment Calendar by dragging your Lesson Plans off the "Learning Plan" for each student. This will reset your student's Learning Plan for the year, so don't do this if your student has already completed assignments because you will lose any record of completion.
After the Calendar is clear, I would recommend auto-assigning only a portion of your lesson plans at a time (use the starting/ending sequence number fields to specify which lesson plan items to assign). This is better for SO many reasons. It gives you greater flexibility for adjusting your schedule, skipping assignments, unexpected sick/vacation days, etc. You can keep assigning more assignments as the year progresses.