This can start getting complicated quickly depending upon your requirements, past usage of the various accounts, and any need you may have for keeping multiple accounts active (such as apps downloaded with both accounts). For example, if you need to use the old account but have apps that were downloaded with the school account, you could set up a brand new email account with a provider (there's plenty that offer free accounts) just so you have a new email address to use with the school account. Then set up Family Sharing with yourself so you can get access to apps and purchases made with the school account (now using a new email address) from the "old account" that is using the email address you wish to have as your Apple ID.
Here's how you change the email address you use for your Apple ID:
1. Go to https://appleid.apple.com and sign in.
2. In the Sign-In and Security section, choose Apple ID.
3. Enter the email address that you want to use as your Apple ID.
4. Choose Change Apple ID.
5. If you changed your Apple ID to a third-party email address, check your email for a verification code, then enter the code. Learn what to do if you didn't get the email. --> If you didn't receive your verification or reset email - Apple Support
6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID. --> What to do after you change your Apple ID or password - Apple Support
For more details, read the document: "Change your Apple ID" --> Change your Apple ID - Apple Support
To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support