To adjust this setting, go to Settings page, and click on the Classes tab. Change the number of hours before lesson and click on Save:

On the student/parent portal, on the Lessons page, it will show a warning text if the lesson cannot be cancelled.
To adjust this setting, go to Settings page, and click on the Classes tab. Change the number of hours before lesson and click on Save:

On the student/parent portal, on the Lessons page, it will show a warning text if the lesson cannot be cancelled.