I am retiring and my apple ID and email will be cancelled by my employer. How do I keep some of the content from apple id?
This is a very hard process to generalize, but we have someone that has written about it exceptionally clearly.

https://www.brianstucki.com/blog/how-to-manually-merge-two-apple-ids-to-one/

The process does take a lot of steps as e@ch bucket of data may or may not be fungible, but I would start with a good backup that you know you can restore before you try any changes. Perhaps you do not even need to merge things, but just change the email from school to home and keep the account, erase the school machines when your retirement day draws close.