I mainly use the newest blender version (4.0) but my school uses 2.79b and can't update from that. What can I do to open files from the newest version to 2.79b
If I remember correctly, since version 4.0 is a major version update, it can and did break compatibility with older versions. I think your best bet is to export your assets individually or the whole scene to another format like OBJ. Then, you can import them in version 2.79b. You may need to do some cleanup afterwards.