This can happen if you edit an event on your calendar that has already been attended, and turn it into a repeating event that extends into the future. since it applies the attendance status to all the events as well.
To remove the “attended” status of the future events, click the next event that is attended, but shouldn’t be, and click “Delete Event”, then click to delete “This and future events”. This will remove the future attended events in that series. Then you can recreate the event series starting on that date.
Another option, if you don’t want to delete the series and recreate it, is to set the attendance for the first lesson that should not be attended to Cancelled. Go to the calendar and open that event in the event edit page, make sure the Cancelled box is checked, and save the event, saving the changes for All Following Events. This will set all of the following events to Cancelled. Then, open that first event again, and remove the check in the Cancelled box, and again save the changes for All Following events. This should reset all the events in the series to be unattended.