OK, you need to find out what file format the documents are in (it will be the three or four letters after the . in the file name, usually .doc or .docx for Microsoft Word)
Also, what kind of "Office" do you have? With year if it has a year. Microsoft Office is the name of a group of programs which includes Microsoft Word. OpenOffice and LibreOffice are free alternatives, ie, they do the same thing but are not made by the same company. They are a perfectly good alternative but teens may initially snub them because they look slightly different. It's worth persevering, make a copy of the work and try opening it and editing it and see if it breaks it or not. Chances are, it won't.
It may be that there are features in the documents which are created in the newest version of Microsoft Word which are not compatible with even the newest versions of Open/LibreOffice.
Microsoft Office is about £100 to buy - you can't get around this. The 365 version still works out at about £100 per year, which when they only bring a new version out every 2-3 years and the latest version should be good for 5-7 years, is a bit of a swizz.
You can use Google Documents to edit and share most Microsoft Office compatible files for free online (Word, Excel, I think Powerpoint too) Again these may not support advanced features.
I think it would be unlikely that school is handing out homework which includes advanced features of Microsoft Word. If they are this is silly because not every pupil will have access to this.
If you have Windows 7 you can install Microsoft Office Starter which is free but comes with ads. Unfortunately it doesn't work on Windows 8 or Windows XP.