You can import Powerpoint, Word and Excel documents into Keynote, Pages and Numbers respectively, and can export to Office formats too.
However, they are not the same and some formatting differences will arise. And without opening (for example) a document created in Pages and exported to Word format in Word, you won't know how it will actually appear. I write reports in Pages but after exporting, an administrative person who receives the document tidies it up in Word before it goes forward to the intended recipients - you probably don't have that facility available.
Also, the latest versions of Keynote, Pages and Numbers are rather more limited in their operation than the previous version (primarily due to Apple seeking to bring the OSX and iOS versions closer together).
If the teachers use Office, and you want to see exactly how the work will appear when they look at it on their computers, then investing in Office for Mac is probably your preferred option.