To back up your mail data from your school account, you can follow the steps below.
1. Open the Outlook app and make sure you are signed in to the school account.
2. Select File > Open & Export > Import/Export.
3. Select Export to a file, and then select Next.
4. Select Outlook Data File (.pst), and select Next.
5. Select the mail folder you want to back up and select Next.
6. Choose a location and name for your backup file, and then select Finish.
You may refer to this link: https://support.microsoft.com/en-us/office/back...
You can then upload this .pst file on your personal OneDrive.
To check which Microsoft account is signed in to your OneDrive app that is showing in file explorer, just follow these steps.
1. Right-click on the OneDrive icon located in the system tray area of the taskbar to open the OneDrive flyout, click on the three dots or gear icon and choose Settings.
2. In settings, choose Account and it will show you which Microsoft account is currently signed in.