I understand your concerns.
In a way there's too much choice. It's like choosing between Facebook or Google Plus. You'll never get everyone to agree on which is best.
I think for the purposes of standardization/control it might be an idea to have one flash card set up for a standard build/project. You may have other cards for experimental use - Different Linux versions, etc.
What I would like to see is some coherent step-by-step learning programme from absolute beginner level.
IE: Lesson 1: Connecting up your R'Pi
It sounds as though Debian is going to be the default OS for starters.
BUT - Who is to say one of the users will come up with some really great Distro that everyone wants? Something compact, with a nice X Window system, with web browser, media player, medibuntu codecs for DVD playpack, lossless wma support, office suite and a bunch of games.
Presumably you will boot up to a Linux command prompt, so some lessons on basic operational commands might be useful: cd, mkdir, cd, cp, mv, rm. Plus a brief description of the Operating System's different folders /home /tmp /var /usr.... and what their purpose is.
Then later on branching out into some very simple programming. Here it will already start to branch out in different directions: Bash Script, C++, Basic, Python, HTML, Java and so on.
In those early days you just had Basic. If you were very clever you might venture into machine code from within Basic. You didn't really have to worry about file structures too much - it was a case of sert the volume to 75% and press play on your tape deck.
So I can see lessons branching out from a tree. The headings might be something like: Networking, File Administration & Permissions, Programming, Multimedia, Web Design, Graphics, Robotics.
What you don't want to happen is that lots of people enthusiastically pay up for a little circuit board. It arrives and they have no clue what to do with it, so it ends up either on Ebay, or shoved into a draw to gather dust.