On the calendar click on the lesson that you want to delete. In the pop-up, click the "Delete" option.
You can delete a single lesson, or if the lesson is part of a repeating series of lessons, then you can delete the selected lesson and all future lessons in the series. Once submitted, the lesson or lessons will be deleted.
You can delete all lessons on a date by going to the Calendar's Month view and hovering your cursor over the triangle icon next to the date number. Click on the Delete link that appears in the pop-up.
You can delete all lessons, all other events, or all lessons and other events on the selected date. Once you click the Submit button, the events will be removed from the calendar.