Eliza - that is a big question, which people write books trying to answer. However, I taught in German "Gymnasium" for five and a half years, which I can say was a pleasure. The pupils were polite, well behaved and generally committed. The staff were friendly, competent and supportive. I was teaching in good schools in the best part of the system, I admit. However, it is worth noting that parts of the school system can still work. (I live in Germany).
My personal view is that what comes out of schools largely depends upon what goes into them. If parents send loved, properly fed and socialised children into schools in the first place, the schools can help them to develop. I do not go along with the philosophy that schools are there to alleviate the damage caused by a lack of - or bad - parenting.