The German school system is complicated - and it doesn't work very well, either. For all those of you who have not had first-person experiences with the system, I'll give a short description first, before going over to the discussion that's been going on - with breaks - ever since 1968 or so.
While most other countries - especially those in industrialized areas - have a rather straight school system with only one type of school for all children (though it's quite probable that they're separated into primary and secondary school). Germany does not. We have one type of primary school and three types of secondary school (not accounting for specialized schools to help the learning impaired). Those three types are completely separated from each other, they are in different buildings, have different head-masters (or -mistresses, of course), different teachers. A child that has been sorted into one of those school types will not, under normal circumstances, switch into another school type later on.
After four to six years of primary school (when I went to school, four years were average, but now primary school usually goes up to sixth grade) the children are sorted into the three types of secondary school. They are between eleven and thirteen then. After this time, they hardly stand a chance to switch into another type of school (at least 'upwards', downwards is always possible). Until the end of their school career (after 9 to 13 years of school overall), they are stuck with this school type.
Now, what are those three types? As I'm quite sure there aren't such types in most other countries, I'll keep the German words for them.
"Hauptschule" once was where the largest amount of students went after primary school (therefore, a lot of primary schools still also function as "Hauptschulen"). It lasts for 9 years (including the aforementioned 4 to 6 years of primary school) and is supposed to prepare them for work in lower-paid jobs in companies. The majority of workers comes from this type of school and it's rather practical orientated.
"Realschule" is the newest type of those schools. It was created when office work became far more important during the industrialization. The school lasts 10 years (again including primary school) and brings forth the middle-class of workers in a company (or at least it's supposed to), so there's more place for theory in this type of school.
The "Gymnasium" (and I know that name is confusing to people from English-speaking countries, as 'gymnasium' there means a building for sports - but both words come from the same Greek origins) is the only type of school from which you can go to university (and Germany doesn't know the principle of colleges). It's as old as the "Hauptschule", while the "Realschule" was created quite some time later. "Gymnasium" lasts 12 or 13 years (depending on the state you live in, southern states usually have 13 years, northern states mostly have 12; again the primary school is included in this number). The "Gymnasium" brings forth the scientists, doctors and high-level personnel for companies. Politicians usually went to the "Gymnasium" as well.
So, now that you know the different types of school, about the discussion in Germany. In PISA we didn't exactly fare well. Even though some states (Bavaria above all) scored at least good enough, Germany on the whole was left in a rather bad position.
But long before PISA, during the 70s for the first time, people tried to change the system. The idea of separating the children that early didn't go well with the young parents who were doing their best, not to rise their children like their parents had. The strict school system was not what they wanted.
Every time they tried to change the system, though, they failed. There are a couple of school types in Germany that incorporate what those parents really wanted to create, but the average public school was not changed. This means, parents who cannot afford or don't really want to sent their children to private schools are left with the old school system and its weaknesses.
While other countries, countries with just one type of secondary school, have fared well with PISA and other tests of that time, Germany still holds on to an outdated school system that keeps children too long in school (if they're going to go to university) or doesn't teach them well enough (if they go to the "Hauptschule").
Politicians always excuse the German system, claiming "it gets better" at the moment and will soon be up to par again, competing with the other countries in Europe, with Japan or the United States. Well, they can dream for all they like, but that doesn't make it true.
The main reason they always give for not changing this system, is money. It's too expensive and won't warrant the costs afterwards, they say. But is it really?
I have to admit that I have no idea about the costs of changing a complete school system - although even I can imagine it will take quite some money. But doesn't the outcome (German children being able to really compete with those from other countries) warrant that money, in the long run. Unfortunately, seeing things developing over a long time, isn't really a forte of German politicians. At the moment, it's too expensive from their point of view (and their children go to private schools, anyway).
The German school system needs a serious make-over, but it won't get it, that much is for sure...