Do these professors state a reason why files need to be in a proprietary formats? Or what would be done (by them to students) if files were to be in a FOSS formats? You can save in the PPTX/XLSX format with LibreOffice suite.
We only need to provide a PDF file so I can use whatever I want.
Ask professors if they know that you have to pay to use MS Office suite. So you can argue that you won’t pay money to be able to make documents/spreadsheets/presentations.