I don't know how German school libraries work, or even how they work in different US states.
But I do know our PA public universities share their 'institutional' access to journal subscriptions with public high school libraries (rather, their Librarians' requests), because tax payor dollars are considered the common denominator.
It's a complicated process that evolved over time. Vocational and Technology schools had to fight for access to studies/publications/journals once reserved for "colleges and universities" only. Traditional Academia was loathe to give up their special status that way, until they were reminded that state taxes not only helped fund their 'institution', but that in-state students were the bulk of their student population.