Using Github for my own project doesn't work

Thanks, but I’m a teacher who has to use GitHub. So no perforce. Being opensource is fine.

I still really need a clue.
How can Unreal tournament be made using GitHub? 100’s of people all using it? There’s 12 of us and we each have a separate folder, so we don’t have to deal with merges. Every lesson it breaks. Files changing by themselves, files locking with no explanation. It’s really really frustrating and is ruining the lessons.
Someone must know what’s happening? With UT and Unreal itself on github??