Add a "Move To" option
The Extend button is great but I think it'd benefit if there is a move feature. You could be able to move a lesson a whole week later. Or if you have something that repeats on every Monday, then you can move it to Monday. When you extend, the lesson would be listed over and over.

Hi there,
You may be able to take advantage of our copy and paste options in this case. When you right click a lesson (from the week view), you should see a few options including Copy. You can select copy, then right click another lesson to choose Paste. This won’t remove the original class, so you would have to overwrite it or delete it. If you have any questions, please email us at support@planbook.com.
Thank you!