For example, my niece initially finished school after 10th grade ("Mittlere Reife"), then did an apprenticeship, then went back to school, working on her abitur

For example, my niece initially finished school after 10th grade ("Mittlere Reife"), then did an apprenticeship, then went back to school, working on her abitur now and will likely go to college afterwards. Yeah, but only few have the discipline to go back to school after having earned "real money" already. The international equivalent would be graduating, getting some work experience and then going back to grad school. I don't think many people manage to do that either. But in this case it's even more difficult because you have to go back to high school basically.