You can use this add-on to add the content of a Google Slides presentation to the end of all the Slides presentations in a Google Classroom Assignment
Welcome to Magic Exercise Book!
This add-on has been specifically designed to use in conjunction with Google Classroom. If you have a Google Classroom Assignment with a Google Slides presentation then you can use Magic Exercise Book to add additional slides at a later date/time.
Magic Exercise Book works from within Google Slides to allow you to copy all of the slides from the current presentation you are working on to the bottom of every students Google Slides presentations within a Google Classroom Assignment.
Essentially you are able to add additional work as you go without having to open each students work and manually copy it across.