Blocking a date does not remove events that were there when you blocked the date. It just prevents new events from being added to it. If you do not want any events on the day, then you should block the dates for the year, before you create the lesson schedules.
If you would like to clear events on those days, here is how: 1. Click the pink blocked symbol in the month view of the calendar to unblock the day. 2. Click on the arrow next to the date. Choose Delete. This deletes all events on the day at once. 3. Click the gray circle in the left corner of the date to reblock the date.