HELP: OSX File Format Fiasco

[MUG NEWS]

Greetings

Need a cure for this OSX Bug:

Bringing files from my classroom of OS X flat panel Macs,
and arriving back at my office with OS/9 machine, all
the stuffit files (.sit) and all graphics files (.gif, .jpg)
have been duplicated into TWO files, one named the same
with underscore in front of the name.

All files are "broken" and cannot be opened.

Stuffit says "This is not a Stuffit file and may be corrupted"

Even Photoshop .psd files which I created here and delivered
there, working perfectly well on the OSX machine, but when
returned here, are broken ... "This is not a Photoshop file"

Is there any way to get this to work.  I need it for grading
class projects.

Isn't it interesting that for so many years we could brag
about the Mac's capability over Windows to open files from
other platforms, but now under OSX the Mac can't even
recognize or open its own files.

:-/

Fred