Hello Tym,
Microsoft 365 admins are equipped to run scans and audits across Microsoft 365 apps.
If the file is saved under your OneDrive, admins are able to access that directly. Be mindful, that OneDrive often sync your Desktop, Documents and Pictures folders by default. If the file is there, and those folders are sync'ed. Your file could be read.
If those files are not sync'ed, and the file is completely offline and only available on your computer. Is it managed? Meaning, does the school require you to enroll your device to their device management systems (like, Intune?) If yes, Admin are most likely capable of auditing your file. While not directly having access, they might be able to read file names and titles.
If the device is not enrolled, and you simply have Microsoft 365 apps installed and logged in with your school, your admins might not be able to have direct access to the files but they again, could audit certain flagged terms, keywords, data used across Microsoft 365 apps.
I hope this answers your questions,
Regards,
Said
