Click on Classrooms under My School and select which classroom you want to update the signature settings for. Click on the ellipsis(. . .) next to the classroom to open the dropdown menu and then click "Edit".

This will bring up the Edit classroom window. You will notice the settings towards the bottom of the window.

If the toggle is green meaning it is enabled, then the signature will be required. If the toggle is gray meaning it is disabled, then the signature will not be required. Click "Save" at the bottom of the Edit classroom window to confirm your updated settings.