Part of the problem might be that the terms are not used in the same way in the Anglosphere and in Germany.

In Germany, the Prussian Reforms refer to what is described in the article and attributed to Wilhelm von Humboldt, this was in the late 18th century.

What you are probably referring to is the Generallandschulreglement by Johan Johann Julius Hecker under Frederick the Great. This was published in 1763, around 40 years before von Humboldt.