Apple IDs use an email address to identify each user. Problems can occur when you can't access that email address anymore:
My Apple ID is associated with my school email which I no longer have access to after leaving the school. How can I change this to my current account without creating problems?
Fortunately, as log as you can still log into your Apple ID account at appleid.apple.com, you can change your email address, thus changing the Apple ID login to that new address. Starting in iOS 10.3, you can make this change directly from an iOS device, too. Apple has full instructions here (https://support.apple.com/en-us/HT202667), and says it may ask you security questions (https://support.apple.com/en-us/HT201485) to validate your identity before the email is sent that lets you confirm a new email address.
Here's some advice to head off this problem beforehand:
Never register something you’re using personally with a school account. You can get a free email account Google and a host of other companies. Always prefer that instead to a company address.
Apple lets you set up a rescue email address, which you can use to reset your security questions if you’ve forgotten them.
And just one more tip, about security questions: password managment apps, like 1Password, can let you capture all the values entered in a form on a web page. I use this feature in 1Password when setting up an account and filling out security questions. Before submitting the form, I use Save New Login from the 1Password web extension, and label it as “Account Name (security questions)”. (I also generate random text to use as answers to security questions, storing those in 1Password, instead of giving out my personal details.)
Source: https://www.macworld.com/article/3222679/data-center-cloud/how-to-change-an-apple-id-email-address-without-access-to-that-email-address.html then adjusted for use in School