When you create a lesson, you can 'lock' one or more elements. Locked elements cannot be moved or modified until they are unlocked.
To lock one or more elements on a slide, follow these steps.
Select the selection/arrow tool (A).
Click the element you'd like to lock.
Click the options button (⋯) in the top left corner.
Click Lock.
Your selected element(s) is/are now locked. You can click on the lock icon again to unlock the element.
Tip: to select multiple elements, you can click and drag a selection area over the desired elements, or click one element and hold Shift while selecting other elements.
Bonus tip: to select and lock all elements on a page at once, use the keyboard shortcuts Ctrl + A (select all) and Ctrl + L (lock).