Then everything from your performance evaluations, the two tests, your physical fitness scores, time in service, time in grade, etc... All are weighted, tallied up, and you find out whether you were promoted or not.
You brought up a good point that acted to help maintain a suitable learning environment when I was in school - if your kid was an ongoing discipline problem at school, your CO was going to hear about it at some point, and he'd "strongly encourage" you to get the kid whatever help he needed, and if you didn't, your odds of promotion on the next cycle were going to be affected. I'm sure things are somewhat different now than they were 45+ years ago when I spent a cumulative seven years in DoDEA schools, but the pressure from above on parents definitely helped to keep the train on the rails, and it's something that is pretty much absent in civilian schools. But from first-hand experience with both, I can say even in the 70s and 80s the difference between the DoDEA and civilian schools I attended was like night and day.