The four primary Vacation periods (Thanksgiving, Christmas, Midwinter, and Spring) each has an Ignore checkbox that will disable the date pickers for each period, and therefore, nothing will be sent by SchoolCal for the ignored dates. (This is easily visualized in the Text Field preview.)
The status of the Ignore checkboxes IS retained between runs, so the next time you launch SchoolCal, you will see the same boxes checked (or unchecked) as you had set for the previous usage.