I’m not entirely certain about the question being asked here.

For questions about Microsoft accounts and Microsoft security and Microsoft apps and services, contact Microsoft.

Also see existing discussions at Microsoft, including: https://answers.microsoft.com/en-us/msoffice/forum/all/it-looks-like-this-email-is-used-with-more-than/1dfbf2e0-58fd-4a00-9a2c-23ea66c74385

If your Proton email is somehow linked to two accounts, I’m unclear how that is being used with that Microsoft Live domain; with (your?) Microsoft Account.

If your iPhone is supervised, assume everything on it is accessible to the folks holding the supervisory lock.

If it is managed, what are the management profiles loaded, and what do those allow access to?

If this iPhone is unsupervised, I’d doubt a school has access into a separate and unrelated email account, as that’d quite possibly end up in court. But whatever might leak into your school account may or will be accessible.