I was cooperatively-schooled (i.e. a small group of parents getting together and taking turns to home-school their children) in England. It was ensured that what we were taught complied with the essentials of the national curriculum (which gave a surprising amount of leeway - I was taught creationist science!) and the authorities were notified of what we were doing. It was very successful.

I don't know about Germany but it's possible that it may be similarly easy to tick the right boxes for the local authorities there too.