The easiest way to make sure that all of your students are opening their documents in Libre- or Open-Office, is to have them change the default program. This is easier than it sounds =) Only 6 steps!
1) Have students click (single-click, not double-click) on any one of their already existing document files, so that the file is highlighted (like the first file below).
2) Then have them hit the "" and "i" keys at the same time. This will bring up the document's "info" window.
3) They should see a section titled "Open with:" as one of many sections available in the Info window.
4) If the section is not expanded, have them click on the side-ways triangle next to "Open with:" and the section will become accessible.
5) Next, have them click on the drop-down menu in the "Open with:" section, and choose either Libre- or Open-Office (which ever you'd like them to use).
6) Last, have them click the "Change All..." button.
This will make sure that all word processing documents will open up in the correct program.
If these steps aren't clear, or you have any questions, let me know, and I'll be happy to attempt clarification. =)