Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Moving from a school-provided Microsoft 365 installation to Office 2019 is mainly a matter of preserving your data, removing the Microsoft 365 license and apps, then installing and activating Office 2019.
If you still have access to your school account, first save any files stored in your school OneDrive to a local folder or a personal OneDrive account. If you used Outlook with your school email, export your mailbox to a PST file before access is removed.
Next, uninstall the Microsoft 365 Apps installation that was licensed through your school. Open Settings > Apps > Installed Apps (or Apps & Features on older versions of Windows), locate Microsoft 365 Apps, Microsoft Office 365, or a similarly named Office entry, and uninstall it.
After the uninstall is complete, install Office 2019 using the installer that came with your Office 2019 license. If you have a retail license associated with a Microsoft account, sign in to that account and obtain the installer from https://account.microsoft.com/services. Run the installer and allow it to complete.
Once installed, open Word or Excel. If prompted, sign in with the Microsoft account that owns the Office 2019 license, or enter your Office 2019 product key if required. After activation, verify the license by opening an Office application and checking Account under the File menu. It should show Office 2019 as the activated product rather than Microsoft 365.
Your Word, Excel, and PowerPoint files do not need to be converted. Office 2019 can open standard .docx, .xlsx, and .pptx files directly.
If your school account has already been disabled and you no longer have access to OneDrive or your school email, any data that was stored exclusively in those services may no longer be recoverable without assistance from the school's IT department.
If the above response helps answer your question, remember to "Accept Answer" so that others in the community facing similar issues can easily find the solution. Your contribution is highly appreciated.
hth
Marcin