Even after submitting the book and/or cover, you as the publisher can cancel the submission. This will enable the school to edit the book again.
Under the Proofing tab, find the school and then cancel the submission by clicking on the X icon next to the book or cover.
Note that after cancelling the submission, you or the book's Editor-in-Chief will need to reset the proofing status to any pages/sections that need to be worked on. Learn how to reject proofs per page, per section or per the entire book.