Copying screenshots, code snipets, buttons, gui etc for educational
purposes does not require the authorization of the copyright
holder. There may be extreme cases where the entire software is
duplicated but I assume you're not refering to them.
If this exceptions to copyright was not well established, pretty much
all educational books about software would be controled exclusively by
the copyright holder of the software. And we know it's not the case, even
without graduating from law school, because of the numerous books written
about the most well known proprietary software.
If this has already been debated and the discussion stalls, I think the
most convincing piece of evidence would be a extract from copyright laws.
IIRC there are also provisions for this in the Berne convention.